34 EVOLUTION 



possibly ancestral types has been found leading back 

 to a remote and very different progenitor. This sup- 

 posed ancestor of the horse was a creature little larger 

 than a moderate-sized dog. It had four separate toes 

 to each fore-limb, and three to each hind-limb, and its 

 teeth were much simpler and less specialized than 

 those of existing horses. We say possibly ancestral, 

 advisedly. Geological evidence is naturally insufficient 

 to establish the actual relationship of the series of 

 types which has been described, and Professor 

 Sedgwick has criticized the view that this series 

 of forms constitutes a demonstrative historical proof 

 of the doctrine of organic evolution. It is, therefore, 

 preferable to claim this group of fossils as an illustra- 

 tion of the possible geological ancestry of an existing 

 species rather than as affording concrete proof of an 

 actual pedigree. Even with this reservation, we claim 

 that such a series constitutes a valuable collateral proof 

 in favour of evolution. The general distribution of 

 organisms throughout the geological strata agrees 

 moreover, in a remarkable way with what is to be 

 expected on the evolution theory. 



6. CHANGES UNDER DOMESTICATION. Among do- 

 mesticated animals and plants we know of numerous 

 cases in which the actual origin of new forms has been 

 observed. These have often differed from their pre- 

 decessors by amounts quite comparable with the dif- 

 ferences by which natural species or even genera are 

 separated. A notable example of this process is afforded 

 by the numerous breeds of pigeons known to have arisen 

 under domestication from a single wild species. We 



