Clock Tutorial #3c - Darwin On Time

This post is a modification from two papers written for two different classes in History of Science, back in 1995 and 1998. It is a part of a four-post series on Darwin and clocks. I first posted it here on December 02, 2004 and then again here on January 06, 2005:

II. Darwin on Time

There is a season for everything
And a time for every purpose under the heaven:
A time to be born, and a time to die:
A time to plant and a time to reap.... (Ecclesiastes)

In this section I will attempt to evaluate from Darwin's writings what he thought about the selective role of environmental periodicities and the adaptive function of internal physiological timing mechanisms ("biological clocks") corresponding to these natural cycles. First, I will present what was published on this subject before and during Darwin's time, e.g., information and ideas that Darwin could have known about. Second, I will try to analyze relevant statements Darwin made in his books and papers in light of the knowledge of his day (non-presentist approach). Third, I will present some key developments in the field since his death in order to understand what he got right, what he got wrong, and ask if he could have done better with what was known at the time (presentist approach).

A Brief History of Biological Time:

Let me first quote some of the earliest records of observations of the influence of environmental periodicities on the living organisms:

"Aristotle [noted] that the ovaries of sea-urchins acquire
greater size than usual at the time of the full moon."(Cloudsley-Thompson
1980,p.5.)
"Androsthenes reported that the tamarind tree..., opened its
leaves during the day and closed them at night."(Moore-Ede et al.
1982,p.5.)
"Cicero mentioned that the flesh of oysters waxed and waned with
the Moon, an observation confirmed later by Pliny."(Campbell 1988, Coveney and
Highfield 1990)
"...Hippocrates had advised his associates that regularity
was a sign of health, and that irregular body functions or habits promoted an
unsalutory condition. He counseled them to pay close attention to fluctuations
in their symptoms, to look at both good and bad days in their patients and
healthy people."(Luce 1971,p.8.)
"Herophilus of Alexandria is said to have
measured biological periodicity by timing the human pulse with the aid of a
water clock."(Cloudsley-Thompson 1980, p.5.)
"Early Greek therapies involved
cycles of treatment, known as metasyncrasis....Caelius Aurelianus on Chronic and
Acute Diseases...describes these treatments.. ."(Luce 1971, p.8.)

Nobody seems to have noticed any biological rhythmicities throughout the Middle Ages. The lone exception was Albertus Magnus who wrote about the sleep movements of plants in the thirteenth century (Bennet 1974).

The first recorded experiment, which is often referred to as the birth of the discipline, was conducted in 1729. by Jean Jacques d'Ortous de Mairan (De Mairan 1729). He shut a heliotrope plant in the basement away from any external light. He noticed that even without the clues from the outside world, the plant opened its leaves by day and closed them again during the night. However, De Mairan was an astronomer busy with questions:

"about the aurora borealis, and the relation of a prism's
rainbow colors to the musical scale, and the diurnal rotation of the earth, and
the satellites of Venus, and the total eclipse of the sun that had occurred in
1706. He would waste no time writing to the Academy about the sleep of a
plant!"(Ward 1971,p.43.),

so his experiment was reported by his friend Marchant.

Henri-Louis Duhamel du Monceau repeated the same experiment several times showing that the sleep-wake cycles are independent of temperature (Duhamel du Monceau 1758). Karl Linne invented his famous flower clock (Linnaeus 1751). Different species of plants were planted in a circle. As each plant opened and closed its flowers at different times of day, a knowledgeable observer could know the time of day by looking at the flowers.

Augustus Pyramus de Candolle showed that the period of diurnal movements of Mimosa is 24 hours in constant darkness, but 22 hours in constant light. He also managed to reverse day and night by using artificial light to which the plants responded by reversing their rhythms (De Candolle 1832). George Newport in 1836 (Newport 1837) observed the behavior of bees, flies and other animals and plants during complete eclipses of the sun. Wilhelm Pffefer, famous for his studies of osmosis in cells and the invention of the clinostat, studied the effects of gravity and light on diurnal movements and growth of plants (Pfeffer 1880,1897,1899,1903,1905). He concluded that these movements are guided by an internal physiological mechanism. On the other hand, Svante Arrhenius, founder of the electrolytic theory of dissociation, attributed the rhythms to the "physiological influence of atmospheric electricity"(Arrhenius 1898).

The list you just read is probably close to complete enumeration of all writings which might bear on the internal biological rhythms which were published before or during Darwin's life and it is likely that Darwin was aware of most of them. Let us now see what Darwin himself had to say on the topic.

Cycles of Selection: What Darwin had to say

Sunrise, sunset, sunrise, sunset,
swiftly fly the years.
One season following another
Laden with happiness and tears.
("Fiddler on the Roof")

There are several mentions of environmental periodicities in the early works of Charles Darwin, but the topic only comes to the center stage in one of his last books, "The Power of Movements of Plants". The earliest hint that seasons should be regarded as selective pressures appears in his Essay of 1859 (Darwin 1859):

"The war, however, is not constant, but recurrent in a
slight degree at short periods, and more severely at occasional more distant
periods..."
"As in every climate there are seasons, for each of its
inhabitants, of greater and less abundance, so all annually breed..."
"Many
practical illustrations of this rapid tendency to increase are on record, among
which, during peculiar seasons, are the extraordinary numbers of certain
animals..."
"Now I think it cannot be doubted that during the breeding-season
all the mice... ordinarily pair..."
"...yet we have every reason to believe,
from what is known of wild animals, that all would pair in the spring."
"It
should always be remembered, that in most cases the checks are recurrent yearly
in a small, regular degree..."
"...average number of individuals...is kept up
by recurrent struggles against other species or against external
nature..."
"...those individuals [being better adapted]..., would be slightly
favoured, and would tend to live longer, and to survive during that time of year
when the food was scarcest..."
"...the struggle of males for the
females....The struggle falls, moreover, at a time of year when food is
generally abundant, and perhaps the effect chiefly produced would be the
modification of the secondary sexual characters,...."
However, the
corresponding passages in the Origin omit any mention of seasons and recurrent
periods of strong selection. Darwin, perhaps, thought that elimination of a
complicating effect of fine temporal scales would make his argument simpler and
easier to absorb by his audience.

Many references to seasonal, lunar and tidal rhythms in the environment and the corresponding adaptations in organisms can be found in the Descent of Man (Darwin 1871), e.g.., :

"Man is subject, like other mammals, birds, and even
insects, to that mysterious law, which causes certain normal processes, such as
gestation, as well as the maturation and duration of various diseases, to follow
lunar periods."
"As famines are periodical, depending chiefly on extreme
seasons, all tribes must fluctuate in number. Hence the progenitors of man would
have tended to increase rapidly; but checks of some kind, either periodical or
constant, must have kept down their numbers, even more severely than with
existing savages.....No doubt, in this case, and in all others, many checks
concur, and different checks under different circumstances; periodical dearths,
depending on unfavourable seasons, being probably the most important of all. So
it will have been with the early progenitors of man."
"Animals can certainly
by some means judge of the intervals of time between recurrent events."
"But
the most curious instance known to me of one instinct getting the better of
another, is the migratory instinct conquering the maternal instinct. The former
is wonderfully strong; a confined bird will at the proper season beat her breast
against the wires of her cage, until it is bare and bloody. It causes young
salmon to leap out of the fresh water, in which they could continue to exist,
and thus unintentionally to commit suicide. Every one knows how strong the
maternal instinct is, leading even timid birds to face great danger, though with
hesitation, and in opposition to the instinct of self-preservation.
Nevertheless, the migratory instinct is so powerful, that late in the autumn
swallows, house-martins, and swifts frequently desert their tender young,
leaving them to perish miserably in their nests."
"We can perceive that an
instinctive impulse, if it be in any way more beneficial to a species than some
other or opposed instinct, would be rendered the more potent of the two through
natural selection; for the individuals which had it most strongly developed
would survive in larger numbers. Whether this is the case with the migratory in
comparison with the maternal instinct, may be doubted. The great persistence, or
steady action of the former at certain seasons of the year during the whole day,
may give it for a time paramount force."
"The inhabitants of the seashore
must be greatly affected by the tides; animals living either about the mean
high-water mark, or about the mean low-water mark, pass through a complete cycle
of tidal changes in a fortnight. Consequently, their food supply will undergo
marked changes week by week. The vital functions of such animals, living under
these conditions for many generations, can hardly fail to run their course in
regular weekly periods. Now it is a mysterious fact that in the higher and now
terrestrial Vertebrata, as well as in other classes, many normal and abnormal
processes one or more whole weeks as their periods; this would be rendered
intelligible if the Vertebrata are descended from an animal allied to the
existing tidal ascidians. Many instances of such periodic processes might be
given, as the gestation of mammals, the duration of fevers, &c. The hatching
of eggs affords also a good example, for, according to Mr. Bartlett (Land and
Water, Jan. 7, 1871), the eggs of the pigeon are hatched in two weeks; those of
the fowl in three; those of the duck in four; those of the goose in five; and
those of the ostrich in seven weeks. As far as we can judge, a recurrent period,
if approximately of the right duration for any process or function, would not,
when once gained, be liable to change; consequently it might be thus transmitted
through almost any number of generations. But if the function changed, the
period would have to change, and would be apt to change almost abruptly by a
whole week. This conclusion, if sound, is highly remarkable; for the period of
gestation in each mammal, and the hatching of each bird's eggs, and many other
vital processes, thus betray to us the primordial birthplace of these animals."
"The cause of this difference between the males and females in their periods
of arrival and maturity is sufficiently obvious. Those males which annually
first migrated into any country, or which in the spring were first ready to
breed, or were the most eager, would leave the largest number of offspring; and
these would tend to inherit similar instincts and constitutions. It must be
borne in mind that it would have been impossible to change very materially the
time of sexual maturity in the females, without at the same time interfering
with the period of the production of the young- a period which must be
determined by the seasons of the year. On the whole there can be no doubt that
with almost all animals, in which the sexes are separate, there is a constantly
recurrent struggle between the males for the possession of the
females."
"With animals in a state of nature, innumerable instances occur of
characters appearing periodically at different seasons. We see this in the horns
of the stag, and in the fur of arctic animals which becomes thick and white
during the winter. Many birds acquire bright colours and other decorations
during the breeding-season alone. Pallas states that in Siberia domestic cattle
and horses become lighter-coloured during the winter; and I have myself
observed, and heard of similar strongly marked changes of colour, that is, from
brownish cream-colour or reddish-brown to a perfect white, in several ponies in
England. Although I do not know that this tendency to change the colour of the
coat during different seasons is transmitted, yet it probably is so, as all
shades of colour are strongly inherited by the horse. Nor is this form of
inheritance, as limited by the seasons, more remarkable than its limitation by
age or sex."
"From the foregoing discussion on the various laws of
inheritance, we learn that the characters of the parents often, or even
generally, tend to become developed in the offspring of the same sex, at the
same age, and periodically at the same season of the year, in which they first
appeared in the parents."
"The coloured plumage and certain other ornaments
of the adult males are either retained for life, or are periodically renewed
during the summer and breeding-season. At this same season the beak and naked
skin about the head frequently change colour, as with some herons, ibises,
gulls, one of the bell-birds just noticed, &c. In the white ibis, the
cheeks, the inflatable skin of the throat, and the basal portion of the beak
then become crimson. In one of the rails, Gallicrex cristatus, a large red
caruncle is developed during this period on the head of the male. So it is with
a thin horny crest on the beak of one of the pelicans, P. erythrorhynchus; for,
after the breeding-season, these horny crests are shed, like horns from the
heads of stags, and the shore of an island in a lake in Nevada was found covered
with these curious exuviae."
"Changes of colour in the plumage according to
the season depend, firstly on a double annual moult, secondly on an actual
change of colour in the feathers themselves, and thirdly on their dull-coloured
margins being periodically shed, or on these three processes more or less
combined. The shedding of the deciduary margins may be compared with the
shedding of their down by very young birds; for the down in most cases arises
from the summits of the first true feathers."
"With respect to the birds
which annually undergo a double moult, there are, firstly, some kinds, for
instance snipes, swallow-plovers (Glareolae), and curlews, in which the two
sexes resemble each other, and do not change colour at any season. I do not know
whether the winter plumage is thicker and warmer than the summer plumage, but
warmth seems the most probable end attained of a double moult, where there is no
change of colour. Secondly, there are birds, for instance, certain species of
Totanus and other Grallatores, the sexes of which resemble each other, but in
which the summer and winter plumage differ slightly in colour. The difference,
however, in these cases is so small that it can hardly be an advantage to them;
and it may, perhaps, be attributed to the direct action of the different
conditions to which the birds are exposed during the two seasons. Thirdly, there
are many other birds the sexes of which are alike, but which are widely
different in their summer and winter plumage. Fourthly, there are birds the
sexes of which differ from each other in colour; but the females, though
moulting twice, retain the same colours throughout the year, whilst the males
undergo a change of colour, sometimes a great one, as with certain bustards.
Fifthly and lastly, there are birds the sexes of which differ from each other
each other in both their summer and winter plumage; but the male undergoes a
greater amount of change at each recurrent season than the female of which the
ruff (Machetes pugnax) offers a good instance. With respect to the cause or
purpose of the differences in colour between the summer and winter plumage, this
may in some instances, as with the ptarmigan, serve during both seasons as a
protection. When the difference between the two plumages is slight it may
perhaps be attributed, as already remarked, to the direct action of the
conditions of life. But with many birds there can hardly be a doubt that the
summer plumage is ornamental, even when both sexes are alike. We may conclude
that this is the case with many herons, egrets, &c., for they acquire their
beautiful plumes only during the breeding-season. Moreover, such plumes,
top-knots, &c., though possessed by both sexes, are occasionally a little
more developed in the male than in the female; and they resemble the plumes and
ornaments possessed by the males alone of other birds. It is also known that
confinement, by affecting the reproductive system of male birds, frequently
checks the development of their secondary sexual characters, but has no
immediate influence on any other characters; and I am informed by Mr. Bartlett
that eight or nine specimens of the knot (Tringa canutus) retained their
unadorned winter plumage in the Zoological Gardens throughout the year, from
which fact we may infer that the summer plumage, though common to both sexes,
partakes of the nature of the exclusively masculine plumage of many other
birds."
"From the foregoing facts, more especially from neither sex of
certain birds changing colour during either annual moult, or changing so
slightly that the change can hardly be of any service to them, and from the
females of other species moulting twice yet retaining the same colours
throughout the year, we may conclude that the habit of annually moulting twice
has not been acquired in order that the male should assume an ornamental
character during the breeding-season; but that the double moult, having been
originally acquired for some distinct purpose, has subsequently been taken
advantage of in certain cases for gaining a nuptial plumage."
"A few words
must be added on changes of plumage in relation to the season of the year. From
reasons formerly assigned there can be little doubt that the elegant plumes,
long pendant feathers, crests, &c., of egrets, herons, and many other birds,
which are developed and retained only during the summer, serve for ornamental
and nuptial purposes, though common to both sexes. The female is thus rendered
more conspicuous during the period of incubation than during the winter; but
such birds as herons and egrets would be able to defend themselves. As, however,
plumes would probably be inconvenient and certainly of no use during the winter,
it is possible that the habit of moulting twice in the year may have been
gradually acquired through natural selection for the sake of casting off
inconvenient ornaments during the winter. But this view cannot be extended to
the many waders, whose summer and winter plumages differ very little in colour.
With defenceless species, in which both sexes, or the males alone, become
extremely conspicuous during the breeding-season,- or when the males acquire at
this season such long wing or tail-feathers as to impede their flight, as with
Cosmetornis and Vidua,- it certainly at first appears highly probable that the
second moult has been gained for the special purpose of throwing off these
ornaments. We must, however, remember that many birds, such as some of the birds
of paradise, the Argus pheasant and peacock, do not cast their plumes during the
winter; and it can hardly be maintained that the constitution of these birds, at
least of the Gallinaceae, renders a double moult impossible, for the ptarmigan
moults thrice in the year."
"Hence it must be considered as doubtful whether
the many species which moult their ornamental plumes or lose their bright
colours during the winter, have acquired this habit on account of the
inconvenience or danger which they would otherwise have suffered."

In the Power of Movement of Plants (Darwin 1880), one of the central topics was the diurnal movement of opening and closing of leaves and flowers ("sleep of plants"). After describing hundreds of experiments performed by his son Fransis and himself, Darwin concludes that the daily rhythms of leaf movements are endogenous and heritable properties of plants shaped by Natural Selection as a means of protection against radiation.

In the Earthworm book (Darwin 1881), Darwin notes the nocturnality of these animals and also describes experiments in which the daily rhythms of activity persisted in earthworms kept in constant darkness. Again, his conclusion was that Natural Selection shaped this innate and heritable trait as an adaptation to avoidance of diurnal predators.

Chronology of Chronobiology: What came after Darwin

It is clear from his writings that Darwin was aware of numerous rhythmic phenomena in the living world, corresponding to the periodicities in the environment. What he did not realize was how connected all those phenomena were. It took several decades after his death until it became clear that sleep movements of plants, nocturnality of earthworms, time-sense of bees, seasonality in flowering and bird coloration, lunar and tidal rhythms in crabs and oysters, migratory orientation in birds, and occurrence of fever in the evening, are all examples of the same phenomenon with the same underlying physiological mechanism - the biological clock.

It is the work of numerous scientists over several decades which laid the foundation for the current understanding that a common mechanism is responsible not just for daily rhythms in all living organisms, but for almost all rhythmic phenomena at different temporal scales.

W.W.Garner and H.A.Allard worked in the early 1920s on control of flowering in a number of cultivated plants. Their results pointed to the existence of short-day and long-day plants, meaning that the plants have the ability to measure the duration of light during the day, thus assessing the season of the year as a critical piece of information needed for the decision to reproduce or not. This was given the name "photoperiodism" (Garner and Allard 1920, 1920a).

Both circadian rhythms and photoperiodism were areas of research for Erwin Bunning, who did his work on the bean plant and, notably, was the first to study Drosophila eclosion rhythms. He proposed the famous "Bunning hypothesis" which states that a circadian rhythm of sensitivity to light is being used to measure how much was daylight "encroaching" into the night-time, thus measuring the photoperiod. It took at least a decade of experimental work by Bunning and others for this hypothesis to get firmly accepted by chronobiologists (Bunning 1936,1958,1973). Today, after the demonstration of involvement of the circadian rhythm in photoperiodic time measurement in so many organisms, the interest seems to have shifted to investigations of those rare species which do not use photoperiod as a seasonal environmental cue (Wingfield et al. 1993, Hahn et al. 1997, Heideman and Bronson 1994, Heideman et al. 1996, Bronson and Heideman 1992).

In 1950's, Gustav Kramer, after many years of researching migration of birds, and a series of ingenious experiments, came to the conclusion that they orient themselves by the sun, correcting for its changing position by checking their internal clocks. This was regarded as a definitive proof of living clocks (Ward 1971, Orr 1970, Berthold 1993, Bonner 1993). At the same time, Ingeborg Beling, a graduate student of Karl von Frisch, showed that honey-bees know the time of day and can use that information for spatial orientation by the sun. E.G.Franz Sauer and Eleanor Sauer put night-migrating birds inside a planetarium and manipulated the image of the night sky to show that some birds can orient themselves by the stars if they consult their internal clocks and photoperiodic calendars (Sauer 1957,1958, Sauer and Sauer 1960).

In 1960., Karl C. Hamner conducted a series of experiments at the South Pole. He monitored the free-running rhythms of bread mold, bean plants, fruit flies, cockroaches and hamsters kept in constant darkness and temperature, on a table revolving in the direction opposite of the rotation of the Earth, the same direction, at half speed and double speed. He has conclusively shown that none of the rhythms was in any way influenced by environmental cues connected to the rotation of the planet. The clocks were most definitely proven to be innate physiological mechanisms.

Patricia Jackson DeCoursey reset the activity rhythms of flying squirrels kept in constant darkness, showing that the clock responds differently to short light pulses depending at what point of the subjective day it is administered (DeCoursey 1960). She plotted all her data relating the amount and direction (advance or delay) of phase-shift to the timing of the pulse on a single curve. This was the first published "phase response curve" (PRC) - the single most important methodological tool in the study of all biological rhythms.

Janet Harker located the first known site of the clock in the eyes of the cockroach, and showed that if the two clocks within two eyes are set out of phase with each other, all the observable rhythms are abolished and the insects develop cancer and soon die (Harker 1960). Beatrice Sweeney found endogenous rhythms in a unicellular organism, a protozoan Gonyaulax polyedra (Sweeney 1960). In 1967. Curt Richter located the site of the mammalian circadian pacemaker in the anterior hypothalamus.

Franz Halberg coined the word "circadian" to denote the rhythms of "about 24 hours". He also did important studies of effects of drugs administered at different phases of the subjective day (Halberg 1964). Jurgen Aschoff did the first research on humans, keeping them in underground bunkers in constant conditions while measuring a host of rhythms including sleep-wake and temperature cycles (Aschoff 1964,1965,1969). Hubertus Strughold in 1965. started the research on the problems of jet-lag (Ward 1971).

The honor of being called "the Father of Chronobiology" goes to Colin Pittendrigh. He was probably the first person to realize the common basis for all the aforementioned phenomena. He was instrumental in gathering together researchers from such disparate fields as plant physiology, animal behavior and human medicine into a new unified discipline. In his 1960. paper, Pittendrigh collected in one place all generalizations about the daily rhythmic phenomena observed in various organisms, demonstrated how such phenomena can be formally studied, and how the interactions between two or more circadian clocks within a single organism can account, at least in theory, for all the biological periodicities and time-measurement mechanisms at all temporal scales. It was a classical case of the use of Consilience as a method. If many weak data from many different fields of study all support the same theoretical model, then it is very likely that the model is correct.

Also, as an evolutionary biologist by training (graduate student of Theodosius Dobzhansky), it was Pittendrigh who, in his work and writings, continuously laid emphasis on adaptive and evolutionary contexts of rhythmic phenomena. He and his students were also excellent examples of fruitful utilization of Feed-Forward, Integrative and Comparative methods in the study of biological timing. Predictions from theory are experimentally tested from various angles in many organisms at all levels of organization, and the resulting data are used to further refine the theory. This style of work has resulted in an explosion of knowledge and understanding of properties of biological rhythms, the underlying mechanisms at all levels of organization, and their evolutionary history.

What you don't know won't hurt you...or will it?

Always listen to experts. They'll tell you what can't be done, and why. Then do it. (Heinlein 1973)

In order to understand how the clocks evolved it is useful to know how they work. At the same time, in order to know how clocks work it is useful to know how they evolved and what kinds of selective pressures have shaped their mechanisms. Except in the "Power of Movement of Plants", Darwin was not interested in answering either one of these two questions. He was interested in the mechanisms of evolutionary change. Daily and other periodicities were interesting only if they could help understand these mechanisms. If much insight could be gained from the role of environmental periodicities on the mode of evolution, then this should be studied in great detail. If internal clocks as adaptations present constraints or dictate direction of evolutionary change, than they are an essential part of the theory and, as such, deserve special consideration. If, on the other hand, the mechanisms of evolution can be elucidated and explained without too much reference to timing, than avoidance of such complicated issues can just add to the clarity of the argument.

So, what did Charles do? In the "Origin", where the main goal was to persuade people that species change and the secondary goal was to propose natural selection as the sole agent of change, references to periodicities are practically non-existent. At the time, Darwin did not know enough about clocks, and did not see a need to invoke timing as part of his argument. Possibility that a change in the properties of an internal timer can lead to temporal isolation and speciation in sympatry could just complicate his forceful arguments for the necessity of geographic isolation. Here, lack of knowledge about proximal mechanisms might have prevented Darwin from introducing too many complicated issues.

There was also no need to brood over seasons or tides when talking about coevolution (Orchids), evolution of sex (Barnacles), heredity (Variation under Domestication), evolution of mind (Expressions), or effects of organisms on the environment (Earthworms). However, both ontogenetic and seasonal adaptations provided a powerful argument for the importance of sexual selection as an alternative evolutionary mechanism (Descent of Man). The fact that so many species of animals in so many phyla display marked sex-specific changes in morphology, physiology and behavior at the age of sexual maturity and only during the breeding season was one of the main phenomena linking sexual reproduction (courtship and mating) to a variety of seemingly non-adaptive phenotypic traits. And it did not at all matter that Darwin treated each specific case separately, showing generality of the ultimate cause without understanding that the underlying mechanism may be common as well. A biologist today, upon reading through thousands of such examples thrown at the reader of "Descent" would just exclaim: "Testosterone!", or, in a little more detail: "Precise timing of surges in sex-specific steroid hormones during embryonic development primes various tissues to respond differently in males and females to photoperiodically induced surges of same hormones at the precise time of the breeding season of a mature adult animal, resulting in sex-specific changes in morphology, physiology and behavior". Darwin would have loved to have known this little fact, as it would have made these phenomena seem easier to evolve, but this did not matter at the time. His argument was powerful enough without references to proximal causes. In this case, data about underlying mechanisms could have made the argument more elegant, but their lack did not make it any less powerful.

In the "Power of Movement of Plants", there was no particular evolutionary mechanism which needed to be presented. Instead, it was an attempt to apply evolutionary thinking to the study of a particular physiological phenomenon. The result is an enormous amount of comparative work, teasing apart generalities from specific differences. It provided the framework for all the future studies on plant growth, movements and biological rhythms. The results of Darwin's experiments paved the way for subsequent studies of proximal mechanisms, but Darwin himself had almost no data, and the book today, apart from inspiration, does not hold much value for plant physiologists.

So, can a pattern be discerned from these three examples? Questions about "evolution of...(a trait or mechanism)" are narrow. Questions about mechanisms of evolution are broad. In the study of a narrow problem (evolution of plant physiology), lack of data was an obvious hindrance. In the medium-sized question (sexual selection), having more data would have been sweet, but not really necessary. For the big question (evolution of life) more detailed understanding of proximate mechanisms would have made the task of discovering and presenting the theory much harder. Sometimes what you don't know hurts you, sometimes it helps you, and at other times does not really matter.

There are other examples of the same pattern. For instance, lack of any knowledge about molecular and cellular mechanisms of inheritance led to Darwin's formulation of a completely wrong theory of pangenesis (Variation under Domestication). He wasn't stupid - his theory was completely consistent with the knowledge about heredity of his time. Yet, that knowledge was so poor that several different theories of heredity could have been formulated and all of them would have been consistent with the known patterns of inheritance. Even awareness of Mendel's work would not have given Darwin enough information to propose a theory of heredity that is really correct. For that, he needed the data from modern genetics, embryology, and molecular biology.

On the other hand, for the formulation of PNS, Darwin needed only the most basic principle of inheritance - "like begets like" - and just the examples which his readers, mostly animal and plant breeders, already knew. One can easily argue that knowledge of all the details of molecular genetics would just hindered Darwin. He would have known too much and would have spent too much time trying to fit the fine points of genetics with his theory. He would have been more vulnerable to critics. It is likely that wealth of information about heredity would have slowed Darwin so much that he would have been scooped - the Principle of Natural Selection (under a different name) would have been discovered by a linguist or an economist. The Grimm brothers were already drawing phylogenetic trees of languages and studied the way they changed over time and branched into new languages. It was a matter of probably just a few years before they, or their colleagues would have hit on the correct answer. Also, it was just a matter of time before somebody put together the doctrine of Malthus with the economic ecology of Adam Smith with the similar result. Of all the systems in which natural selection operates, the organic world is the most complex. It speaks for the genius of Charles Darwin that he got the PNS first, before anybody else figured it out in a simpler system. Also, he was fortunate that great lack of knowledge of the time simplified his system to the extent that he was able to discover the evolutionary mechanism anyway.