Darwin, Charles Robert (1809–82)

2. The evolutionary worldview

Conventional histories of the Darwinian revolution have assumed that, prior to the publication of the Origin of Species, everyone accepted that each species was designed by a wise and benevolent God. Darwin’s materialistic theory thus upset a vision of a perfectly ordered universe. In fact, the idea of evolution had been widely discussed in the previous decades. Robert Chambers’ Vestiges of the Natural History of Creation (1844), promoted the image of life progressing towards higher levels of physical and mental organization under the control of divinely implanted laws. Historians still debate the extent to which Darwin himself saw evolution as necessarily progressive. There was certainly an element of progressionism in his thinking, but his theory was built on different foundations and in later life he was unable to accept that the whole process was designed by the Creator. Darwin may have begun his work believing that the laws governing natural development were instituted by God, but he ended up seeing the universe as a scene of perpetual struggle and suffering.

Darwin argued that each population changes by adapting to its local environment. When members of a population migrate to new areas, they will adapt to the new conditions and thus come to differ from the parent group. If the degree of difference becomes sufficient, the two populations would find it difficult to interbreed even if they came into contact, and would thus count as distinct species. Local varieties or subspecies are the first steps on the way to full speciation. The overall pattern of evolution must thus be visualized as an irregularly branching tree, not as the ascent of a ladder towards a fixed goal.

Darwin believed that evolution was an immensely slow process, and thus that it was unrealistic to expect ever to observe one species being transmuted into another. Opponents of the theory argued that we only observe minor changes within species, from which it is illegitimate to infer that change spead over a longer timespan will generate a new species distinct from the parent form. Darwin realized that the case for evolution would have to be argued by showing that the theory accounted for a range of otherwise inexplicable phenomena. He showed that the classification of species could be understood as a reflection of the tree-like model of genealogical relationships (see Taxonomy §§3–4). Each major taxon was founded by a single ancestral form which had developed some important evolutionary innovation; the descendants of this ancestor radiated out through the local adaptation of sub-populations, thus creating the genera and species we recognize today. Homologies such as the similar bone structures of the forelimb in humans, horses and bats were explained as adaptive modifications of similar structures inherited from a common ancestor. Relationships between dissimilar adult forms could often be traced in their early embryos, which were not subject to the same degree of modification. Darwin knew that evolutionary change could not be demonstrated from the fossil record, which often gave the appearance of sudden ’leaps’. He argued that the record is very imperfect, the ’leaps’ are in fact gaps concealing gradual evolutionary transformations. Once this factor was taken into account, the fossil record could be reconciled with the tree model. The geographical distribution of organisms could also be explained in terms of species migrating outward from their point of origin, adapting themselves to the new territories they entered (see Evolution, theory of).