CLASSIFICATION AMD EVOLUTION 



523 



stages, and that animals which are more alike resemble one 

 another longer during development. All animals have at 

 one stage no body nuclei, all Metazoa have at a later stage 

 two layers only, all Vertebrata at a still later stage have gill- 

 clefts and a notochord, all mammals at a later stage yet 

 are five-fingered, and so forth. The simplest explanation 

 of these facts is that (for reasons into which we cannot 

 enter) development is a very rough recapitulation of 

 evolution. This deduction from von Baer's law is known 

 as the theory of recapitulation. (4) The facts of distribution 

 are yet another support for the theory of evolution. It is 





Fig. 389. — The gradual transition between Paludina neumayri 

 (a), the oldest form, and Paludina hcernesi (_/). — From 

 Neumayr. 



found that the animal populations or faunas of various 

 parts of the world differ, that this is the case even when 

 the climates of the two regions are so similar that animals 

 native in one will flourish in the other, as in England and 

 New Zealand, and that the difference increases with the 

 inaccessibility of one from the other. These facts have no 

 explanation in the theory of special creation, but are easily 

 explained on the supposition that the course of evolution 

 has been different in the two cases owing to different 

 histories in past geological times. (5) The facts of 

 paltzozoology (or the geological history of animals) are also 

 in favour of evolution. It is clear that this is the only 

 direction in which we could look for a complete proof 'of 

 the theory, since all the other evidence does no more than 



