8o THE EVOLUTION OF ANIMALS. 



was not received with much favour till Darwin, in his " Origin 

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

 his work and by that of Spencer, Wallace, Hseckel, 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, (p) physiological, (c) historical. I shall not restate 

 the arguments, but content myself with reminding you of 

 their nature. 



Evidences of Evolution. 



(a) Structural— {\) There are said to be about two 

 million living animals of different species, that is to say, of 

 different kinds which are not readily fertile with one another. 

 These species are connected together by varieties, which 

 make strict severance often impossible ; they can be ration- 

 ally arranged in genera, orders, families, and classes, between 

 which there are sometimes 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 genealogical tree. A little practical 

 experience makes one feel that the facts of classification 

 favour the idea of common descent. 



(2) 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. 



(3) 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, or of embryonic parrots, the pineal body 

 (a rudimentary eye) in Vertebrates, etc., etc. Only on the 



