I06 THE DOCTRINE OF DESCENT 



of Species (1859), made it current intellectual coin. By 

 his work, and by that of Spencer, Wallace, Haeckel, Huxley, 

 and many others, the doctrine of descent, the general fact 

 of evolution, has been established, and is now all but 

 universally recognised. 



The chief arguments which Darwin and others have 

 elaborated in support of the doctrine of descent, according 

 to which organisms have been naturally evolved from 

 simpler forms of life, may be ranked under three heads — 

 {a) structural, (b) physiological, {c) historical. 



Evidences of evolution. — {a) Structural. — Some say 

 that there are over a million living animals of different 

 species. In any case, there are many myriads. These 

 species are linked together by varieties which make strict 

 severance often impossible ; they can be rationally arranged 

 in genera, orders, families, and classes, between which there 

 are not a few remarkable connecting links ; there is a 

 gradual increase of complexity from the Protozoa upwards 

 along various hues of organisation ; it is possible to rank 

 them all on a hypothetical genealogical tree (Fig. 18). A 

 little practical experience makes one feel that the facts of 

 classification favour the idea of common descent. 



Throughout vast series of animals we find in different 

 guise essentially the same parts twisted into most diverse 

 forms for different uses, but yet referable to the same funda- 

 mental type. It is difficult to understand this " adherence 

 to type," this " homology " of organs, except on the theory 

 of natural relationship. 



There are many rudimentary organs in animals, especially 

 in the higher animals, which remain very slightly developed, 

 and which often disappear without having served any 

 apparent purpose. Such are the " gill-slits " or " visceral 

 clefts " in Reptiles, Birds, and Mammals, the teeth of young 

 whalebone whales, the pineal body (a rudimentary eye) in 

 Vertebrates. Only on the theory that they are vestiges of 

 structures which were of use in ancestors are these rudi- 

 ments intelligible. They are relics of past history, com- 

 parable, as Darwin said, to the unpronounced letters in 

 many words. 



(b) Physiological. — Observation shows that animals are to 

 some extent plastic. In natural conditions they usually 



