DARWINISM AND EVOLUTION DEFINED. 17 



natural selection. But the assumed choice in the theory 

 of sexual selection has a much less mechanical and auto- 

 matically working basis, involves violent assumptions re- 

 garding the aesthetic development of birds, butterflies, and 

 spiders, and as we shall later see was one of the first of Dar- 

 winian outworks to be sadly breached by attack. 



I hope now to have pointed out clearly in the preceding 



paragraphs the real distinction between the theory of 



descent and the theory of natural selection 



Theory of de- (Darwinism). The bases, consisting of ob- 

 scent and the 



theory of natural served facts and logical reasons, of the selec- 

 tion theor y> have been given; perhaps it were 

 well to state briefly the bases, or sources of the 

 scientific evidence for the theory of descent. This evi- 

 dence is derived from three chief sources; the study of the 

 comparative anatomy and structural homologies of organ- 

 isms, the study of the prehistoric animals and plants, that 

 is, palaeontology or historical geology, and the study of 

 ontogeny, or embryology, that is, the development of in- 

 dividual animals and plants. The homologies or structural 

 correspondence, in gross and in detail, which the study of 

 animal and plant comparative anatomy reveals to exist in 

 varying degrees among living and extinct kinds of organ- 

 isms have but one possible scientific explanation : an explana- 

 tion which serves at once to account for the existence of 

 this correspondence and for its varying degrees. 

 This ex P lanation is community of ancestry, the 

 blood-relationship of organisms, the theory of 

 descent. Similarly the facts revealed by the study of 

 palaeontology are explicable wholly satisfactorily by the 

 theory of descent and in no single definitive instance do they 

 contradict it. Finally, the facts and conditions relating to 

 the embryology or ontogeny of animals and plants are 

 similarly wholly in consonance with the theory of descent, 

 although the brilliant positive evidence for the theory which 



