WHAT IS A SPECIES? 157 



call species, i.e., the descendants of common parents, vary 

 among themselves, and that the variability is substantially 

 universal. This was elaborated by study of the variation of 

 plants and animals, and particularly of pigeons under domesti- 

 cation. The selection which man makes in his stock-breeding 

 suggested to Darwin the idea that the very conditions of 

 environment would act in the course of ages as selecting 

 agencies, favoring the growth and transmission of certain 

 peculiarities of structure or habit, and working against other 

 varietal characters, thus eventually perpetuating the favorable 

 varieties, and causing ill-adapted characters to become lost. 

 Undoubtedly his observations, when a boy, of the results of 

 stock-breeding among Leicester sheep and the ideas of Mr. 

 Bakewell, with whom he was acquainted, impressed them- 

 selves upon his memory and were the foundation of the 

 theory, the elaboration of which made him famous. 



The Evolution Theory of Biology and the TJniformitarian 

 Theory of Geology. — Darwin's " Origin of Species " brought the 

 world to a vivid appreciation of the universal mutability of 

 all organic things, and the theory which bound together the 

 mutability of organisms was evolutionism. It is interesting, 

 from a philosophical point of view, to note that about fifty 

 years before, a like step of progress was reached through the 

 uniformitarian theory of Hutton, which set forth the principle, 

 that during all geological time, there has been no essential 

 change in the character of geological events; but uniformity 

 of law and conservation of force are perfectly consistent with 

 the mutability in the results and the incessant evolution of 

 present life out of the dying past. 



Evolution and Development Contrasted. — In its general sense 

 I find no better definition of evolution than that given by 

 Huxley: " Evolution or development,'' he says, " is, in fact, at 

 present employed in biology as a general name for the history of 

 the steps by ivhieh a)iy living being has ae quired the morphologi- 

 cal a7id the physiological characters which distitigtiish it.'' 

 Evolution, as has been already noted, in this sense confuses 

 two processes which may co-operate in the result, but which 

 may be distinguished in their exhibition in actual facts of the 

 history. They are technically separated under the two cate- 



