Inferring and Explaining

75 Men evolved from apes, and apes from more primi- tive animals, and the primitive animals evolved from the soup of inanimate atoms which consolidated to form the Earth some four thousand million years ago. Although there is much uncertainty about the exact stages andmechanisms involved, the fact of evolution is evident. 15 Natural Selection Why do we see such change in biological his- tory? Why does this change so often seem exactly what is required for changing circum- stances? What is the engine that drives descent with modifcation? In considering the Origin of Species, it is quite con- ceivable that a naturalist, refecting on the mutual afnities of organic beings, on their embryological relations, their geographical distribution, geologi- cal succession, and other such facts, might come to the conclusion that each species had not been inde- pendently created, but had descended, like varieties from other species. Nevertheless, such a conclu- sion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modifed, so as to acquire that perfection of structure and coadapta- tion which most justly excites our admiration. 16 One of my teachers called natural selection an algorithm, and Ernst Mayr, who I will be bor- rowing heavily from in this section, calls the theory “very logical.” 17 All this could be taken to mean that natural selection is automatic or that the inference is deductive.Te argument, though, is explanatory, just like the argument for common descent. I will followMayr, indeed I will use his wording, and treat Darwin’s reason- ing as “three inferences based on fve facts.” 18 Tree of these facts provide evidence for what he called the “struggle for existence.” A struggle for existence inevitable follows from the high rate at which all organic beings tend to increase. Every being which during its natural lifetime pro- duces several eggs or seeds, must sufer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geo- metrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must be in every case a struggle for existence, either one individual with another of the same species, or with distinct species, or with the physical conditions of life. 19 darwIn and Common desCent e 7 . All species have such great potential fertility that their population size would increase exponentially . . . if all that are born would again reproduce successfully. 20 e 8 . Except for minor annual fuctuations and occasional major fuctuations, populations normally display stability. 21 e 9 . Natural resources are limited. In a stable environment, they remain relatively constant. 22 t 1 0 . There is a ferce struggle for existence among individuals of a population, resulting in the survival of only a part, often a very small part, of the progeny of each generation. 23 Te game of life is unfair—not all compete in this struggle for existence equally. Darwin, who was a pigeon breeder and recognized its rele- vance tohis argument, sawthat some individuals