32 EVOLUTION 



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 

 1eeth were much simpler and less specialized than 

 those of existing horses. 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 

 have no reason whatever for supposing that domesti- 

 cated species are more mutable than wild species, and 

 there is consequently every reason to believe that 

 changes of a similar character take place in Nature. 

 The conditions of domestication, of course, afford much 

 better opportunities of observing such phenomena. 



7. THE OBSERVED FACTS OF MUTATION. Neverthe- 

 less, individual specimens of particular wild species are 

 frequently found showing modifications which, if they 

 occurred constantly in an isolated group, would afford 

 a basis for the description of new species. In a few 

 cases the actual occurrence of similar changes has been 

 observed in wild species of plants. 



