82 THE DOCTRINE OF DESCENT. 



was not received with favour until Darwin, in his " Origin 

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

 his work and by that of Spencer, Wallace, Haeckel, 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) struc- 

 tural, (b) physiological, (c) historical. 



EVIDENCES OF EVOLUTION. 



(a) Structural. There are said to be over a million liv- 

 ing animals of different species. These species are linked 

 together by varieties, which make strict severance often 

 impossible (Fig. 15); 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 lines of organisation ; it is possible to rank them 

 all on a hypothetical genealogical tree (Fig. i). A little 

 practical experience makes one feel that the facts of classi- 

 fication 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, espe- 

 cially 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 rudiments 



