EVIDENCES OF EVOLUTION. 105 



Evidence of Organic Evolution. 



After this long preamble respecting classification 

 and species a preamble which the nature and scope 

 of the topic now under discussion have rendered 

 necessary we are at length prepared for an intelli- 

 gent appreciation of the arguments commonly ad- 

 duced in support of the theory of organic Evolution. 

 If species are not the immutable units they have so 

 long been considered ; if, far from being easy of rec- 

 ognition, as is so often fancied, they are with diffi- 

 culty recognizable, if at all ; if, far from being perma- 

 nent and unchangeable, they are, on the contrary, 

 variable and mutable ; we have legitimate a priori 

 reasons for believing in the possibility of Evolution, 

 if not in its probability. The actuality, however, 

 of Evolution, is a question of evidence ; not indeed of 

 evidence based on metaphysical assumptions, but of 

 evidence derived from observation and a trustworthy 

 interpretation of the facts of nature. To the discus- 

 sion of this evidence, which I shall make as brief as 

 is consistent with clearness and the nature of the 

 argument involved, I shall now direct the reader's 

 attention. 



The evidence usually advanced in support of 

 organic Evolution is fourfold, and is based: First, 

 on the classification of animals and plants ; second, 

 on their morphology ; third, on their embryology ; 

 and fourth, on their distribution in space and 

 time. This, especially the evidence derived from 

 paleontology, is what Huxley designates as "the 



