Some Philosophical Aspects of the Theory of Evolution

on num'Ull.ues, and even on has been one of the most over- .r.,ng:m,n ~-~~M,-~,",-' It has been used *Read at a meeting of the llolloqulU:m in the Humanities, Unllver'Sltv 386 THE THEORY OF EVOLUTION 387 ary process as a whole has a "direction" and hence is not altogether "random"? The consideration of these questions will oblige us to distinguish issues which are of a factual nature, decidable on observational grounds, from issues which are either matters of interpretation or else are linguistic and verbal. I shall try to show inter alia that many misunderstandings have had. their origin in a failure to make these distinctions explicit. I At the outset it will be well to remind ourselves that the theory of evolution is not a simple doctrine which can be formulated in a few sentences. Like most fundamental scientific theories it has a complex structure, and is continually being modified in its details. One can, however, distinguish two parts within the theory. The first part seeks to describe the actual history of living things on the earth. This descriptive task is undertaken in the main by palaeontology and palaeobotany , and utilizes the evidence of fossils. The findings of these sciences permit the drawing of certain conclusions as to the course which evolution has in fact followed. The second part of the theory seeks to give a causal explanation of the evolutionary process, utilizing for this purpose evidence drawn from the contemporary study of living things plus various hypotheses of a relevant sort. With regard to evolution as a historical process, it is supported by such a vast array of considerations that no infonned and reasonable person is likely to doubt that it has occulTed. With regard to the causal explanation of this process, however, there is much that remains conjectural and the subject of legitimate disagreement. Both these aspects of the theory) the historical and the causal, were brought together in an impressive manner by Charles Darwffi in The Origin of Species. By the time this work appeared, most scientific men were becoming convinced of the truth of certain conclusions about the plants and animals on the earth. Geological investigation of the temporal order of different strata of rock on the surface of the globe, together with the discovery of fossil remains of organisms in those strata, made it fairly certain that there had been a succession of living forms on the earth for an immensely long period of time. Hence, it was concluded, all organisms now living must have descended from other organisms, often very different in character, which lived in the remote past. Furthennore , the record seemed to show that many of the differences involved had arisen gradually through the ages, so that if all the individuals which had ever existed could be assembled, a broadly continuous array of forms would be found. Thus the old, sharp lines of 388 UNIVERSITY OF TORONTO QUARTERLY successive ess(~ntlaJ.s~ the The THE THEORY OF EVOLUTION 389 already established, he made a further inference, namely, that the variations which assist individuals to survive and reproduce will tend to' be preserved in the species, while variations unfavourahle to sur~ vival and reproduction will gradually be eliminated. To this process he gave the name "natural selection"-another metaphor, incidentally , but one which was often construed both by himself and by others in a completely literal sense. Since many variations are undoubtedly transmitted from parents to offspring, natural selection will produce, he contended, a slow accumulation of favourable variations in a species from generation to generation. Accordingly, a species possessing such variations will tend to become better and better adapted to a given way of life, or will shift gradually from one way of life to another. In either case, changes will take place in the observable features of the members of the species in successive generations, so that ultimately a "new" species will be produced. Natural selection is, therefore, Danvin concluded, the main cause of the origin of species and of the historical course taken by the evolution of living things on the earth.4 DaIWin's causal account of the evolutionary process is thus...