Darwin’s Forgotten Metaphors
Natural selection, Darwin’s most novel metaphor, was the key concept in distinguishing Darwinian selectionism from Lamarckian adaptationism. Darwin therefore devoted the bulk of Origin of Species to elaborating the concept. As well, even though Darwin viewed natural selection as an emergent property of the nature of the organism, mechanisms underlying the inherent properties of organisms were not understood (Darwin referred to the mysterious laws of growth, the laws of inheritance, and the laws of the correlation of parts without specifying mechanisms to explain them). As a result, natural selection was the concept most amenable to normalizing language and became the primary focus of neo-Darwinism.
The Conditions of Existence
Darwin proposed that biological diversity emerged from
…two factors: namely, the nature of the organism and the nature of the conditions. The former seems to be much more the important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform. (Darwin 1872: 32)
Darwin’s conception of the “nature of the organism” was explicit, even if he lacked mechanisms of inheritance and ontogeny. Darwin understood that organisms were historically and developmentally cohesive wholes, and therefore it was in the nature of the organism to produce offspring that were all highly similar (but not identical) to each other and to their parents and other ancestors. He also postulated that reproduction produced variation without regard for environmental conditions, and therefore it was in the nature of the organism to produce these offspring in numbers far exceeding the resources available for their support. This was Darwin’s Necessary Misfit (Brooks and Hoberg 2007; Brooks 2011a, b).
Organisms cannot produce more offspring than there are resources to support them in a Panglossian (Lamarckian) world, so there must be constraints on responses to the surroundings. Darwin resolved this conundrum by postulating that the nature of the organism, in the form of insensitivity to the nature of the conditions, created the constraints. And yet, those constraints were not absolute. All surviving organisms have positive Darwinian fitness, but some are fitter than others in the environment in which they were produced. These tend to predominate numerically over their merely adequate relatives. Those relatives, however, survive and play a decisive role in Darwinian evolution. Whenever an environment changes, the fittest organisms in the old environment might not survive at all in the new, whereas some of the merely adequate in the old environment might have the adaptations necessary to survive, and even flourish, in the new one. Natural selection was thus an emergent property of the inevitable conflict created by the conditions of existence and was a metaphor for the ways to resolve such conflicts, setting the stage for resolution of conflicts yet to come. More than 150 pages after introducing the duality of the conditions of existence, Darwin called it the higher law of biology, underscoring the emergent nature of natural selection:
It is generally acknowledged that all organic beings have been formed on two great laws—unity of type and the conditions of existence.…On my theory, unity of type is explained by unity of descent. The expression [my italics] of conditions of existence…is fully embraced by the principle of natural selection.…Hence in fact the law of the Conditions of Existence [my italics] is the higher law; as it includes, through the inheritance of former adaptations, that of Unity of Type. (Darwin 1872: 194–195)
The Complexity of Evolution
Darwin proposed two rich visual metaphors to help readers understand the fundamental complexity of evolution.
The Phylogenetic Tree
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications. (Darwin 1872: 000–000)
The Tree of Life metaphor is more than an accounting scheme; it is a symbol of a major part of the evolutionary process. Living systems are capable of acting in their own behalf, but more importantly, they regularly take the initiative, using what they have inherited. Metaphorically, the present is the state in which biological systems create their own futures based on their own pasts. Organisms carry so much of their history with them that most explanations for their appearance and function stem from their past—this is the focus of historical ecology (Brooks and McLennan 2002). Specific points of origin in space and time play integral roles in explaining the properties of species and the organisms that comprise them, most importantly how they interact with their surroundings, including other species. I recently spent a year in Europe, where a “sycamore” is a maple (Acer pseudoplatanus) and a “plane tree” (Platanus orientalis) is what I call “sycamore” (Platanus occidentalis). Darwin’s metaphor of natural classification being a phylogeny enables us to understand why North American sycamores and European plane trees resemble each other so closely, why their ecological preferences are so similar, and why they are able to hybridize so readily.
Darwin’s phylogenetic tree metaphor contrasted with a progressive view of diversity embodied in the Scala naturae, in which “lower forms” were replaced by “higher forms.” Thus, the only illustrated metaphor Darwin ever provided in any edition of Origin of Species specifically underscored the notion of evolution as one of selective accumulation of diversity rather than selective replacement.
The Tangled Bank
It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. (Darwin 1872: 000–000)
This is clear and a lyrically metaphorical statement which evokes visions of selective accumulation of diversity producing complex ecosystems. It also explicitly underscores Darwin’s view that natural selection is an emergent property, reinforcing his conception of the Law of the Conditions of Existence.