Variety is the spice of life

Throughout high school and college I was taught the same thing about the history of science. Young earth creationists had a stranglehold on explanations for life's origins until the fateful year of 1859 when Charles Darwin convinced all but the most ardent fundamentalists that evolution by means of natural selection was a reality. It is a neat and tidy story, a tale in which one book changes the world forever, and it is completely wrong.

As I started to dig deeper into the history of science I found a complex story which I had never heard about before. Evolutionary ideas were percolating among naturalists well before 1859, and Darwin certainly did not have the last word on how organisms evolved. Indeed, there is a great span of time, from 1860 through about 1950, which is often ignored in popular summaries of evolutionary thought. That is because those are the years between the publication of On the Origin of Species and the establishment of the modern evolutionary synthesis, a time during when the mechanisms by which life evolved were in doubt.

[The historian Peter Bowler has published two accessible summaries of this time, The Eclipse of Darwinism and The Non-Darwinian Revolution. Additionally, for a discussion of evolutionary science in Victorian England bracketing 1859 see Adrian Desmond's Archetypes and Ancestors.]

I find this time especially interesting. Darwin's work had legitimized evolution as a topic worthy of study, but there was disagreement about what caused life to change. Practitioners of different disciplines forwarded various explanations, and all of them carried biases stemming from the kind of evidence being examined to parse the "mystery of mysteries." Paleontologists, for instance, often thought that the patterns they saw in the fossil record were more in accord with a kind of evolution that was internally-driven (even if they could not explain the details of how traits forced by such a mechanism became inherited). Figuring out the pattern took precedence over figuring out the details of the process, and I ran into this again in Frederic A. Lucas' 1902 book Animals Before Man in North America.

The majority of Lucas's book consists of a standard treatment of paleontology and what life was like during different slices of the prehistoric past. What caught my attention was in the conclusion. After surveying the succession of fossil forms in North America, Lucas finally addresses what might have caused what he interprets as the "onward and upward" progression of life. He writes;

There may not be an agreement as to the reason for these changes, and it might be well to frankly admit that what are styled causes are really only carefully framed theories which seem to account for observed facts. But to sum up in a very few words, it may be said that there seems to be an inborn tendency in living things, both plants and animals, to vary and to adapt themselves to circumstances. Changes in their surroundings--and these are ever taking place--simply allow this natural tendency a chance to act. The simpler the creature and the more uniform the surroundings, the less would be the tendency to vary. The more complex the structure of an animal and the more variable the conditions to which it was subjected, the more liable would it be to undergo some change. And as more and more highly organized animals appeared on the scene the more rapidly would changes take place.

Though this sounds similar to evolution by natural selection, Lucas' phrasing suggests something a bit different. According to Lucas, who would later attribute this idea to another naturalist named Cook, each organism has an internal, or "inborn tendency" towards particular variations that can be enhanced or stifled by the environment. New characteristics are not so much being selected for as being allowed to be expressed. This can be seen in the next paragraph in which Lucas writes;

Thus, some of the simple animals that dwell in the depths of the sea, where quiet, darkness, and cold prevail, have a history that reaches back into the past for lengths of time almost inconceivable to us, amounting to millions of years. On the other hand, none of the mammals now living are at all nearly related to those that nourished during the period of time we call Eocene, while few, indeed, are to be found even in the Pliocene. And that mammals should have changed more rapidly than any other animals is only what might have been expected from their high organization, as this should theoretically render them particularly susceptible to changes going on about them.

This suggests that Lucas thought that it was the organisms themselves that were progressing towards particular forms and the environment either perpetuated this trend or held it back. Populations of organisms were not becoming adapted to certain conditions over time but were themselves adapting to local conditions. That Lucas was vague on this subject makes it somewhat difficult to precisely parse his meaning, but in my reading he favored an evolution mechanism driven from within rather than one that required forces imposed from without.

Yet Lucas recognized at least two problems organisms would face if his hypothesis were true. One was that "life's race" might become so intense that even complex animals could not keep up with the shifting environment and sink into extinction. This was the fate of the strange mammals that lived during the Eocene, or the beginning of the "Age of Mammals." They were so complex that they were somewhat unstable and driven to extinction by environmental changes they could not adapt to. The other danger was that a group might end up being suppressed for very long spans of time if the environment was not amenable to the direction a group's evolution was being pushed towards, as with mammals in the age of dinosaurs. In Luca's view, it would take the extinction of the dominant archosaurs for the mammals to realize their evolutionary potential.

I find Lucas's hypothesis especially interesting because he traded in phrases associated with natural selection such as "the struggle for existence" but he appeared to favor a more internally-directed evolution. Ultimately, though, this vision of evolutionary progress would fall out of favor. Individual organisms did vary, but these variations were not unidirectional; organisms are not striving to reach the next rung on an evolutionary ladder. Understanding this has made all the difference.