GEOGRAPHICAL DISTRIBUTION III 



mation of the five-toed limb of a form like Phenacodus into 

 the one-toed limb of the horse (Plate 47). Notice also the 

 increasing complexity of the ridges on the grinding surface 

 of the teeth of the same species from which the illustrations 

 of foot structure are taken. We have here a very complete 

 paleontological record of a profound change of structure, 

 giving us the actual history of the evolution of the horse. 



Geographical distribution. 



The comparison of the phenomena of paleontology, 

 anatomy, and embryology seems to point us very clearly 

 to the theory of evolution as the solution of the problem 

 of origin. It is interesting also to find that the distribution 

 of animals and plants over the earth is such as this theory 

 would lead us to expect. We find the character of the 

 fauna and flora decidedly different in different regions of 

 the earth, and these differences are not due solely to differ- 

 ences of climate and soil and other conditions of the envi- 

 ronment. Similar environmental conditions do not produce 

 similar animals and plants if the regions compared be sepa- 

 rated from each other by sufficient distances or by barriers 

 that prevent free migration and interbreeding. The phe- 

 nomena of distribution, as we find them, agree with the 

 hypothesis that the different species of animals and plants 

 have each arisen at some particular place and have spread 

 from that spot, becoming modified to a greater or less 

 extent during their wandering. 



In general, we may say that the degree of intimacy in 

 relationship between the faunas and floras of any two 

 regions is in inverse ratio to the degree to which barriers 



