654 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Probable as this conclusion is made by all the available evi- 

 dence, it cannot be regarded as demonstrated ; it is prover- 

 bially impossible to prove a negative. 



On the other hand, it is equally probable that nearly related 

 forms do very frequently, perhaps normally, pass through 

 separate, but closely similar, courses of development. It is 

 likely that a new species is usually formed through similar 

 and simultaneous modification of many individuals, rather 

 than from a single individual or pair. It may be the general 

 rule, as almost certainly has often happened, that a new genus 

 arises by the separate assumption of the new character by several 

 species of the ancestral genus, rather than through the rapid 

 diversification of a single species, though, no doubt, parallel 

 and divergent modification are both very frequent and im- 

 portant processes. Dr. Eigenmann concludes from his study 

 of South American fresh-water fishes that a certain new genus 

 is even now in process of origin through the transformation of 

 several species of an older genus, which in different parts of 

 the continent are simultaneously, but independently, taking 

 on the new character. 



Sometimes it is possible to assign a definite reason for the 

 independent origin of similar structures in different groups of 

 mammals. Except for the head, there is much similarity 

 of appearance among the very massive hoofed animals, such 

 as the elephants, rhinoceroses, tapirs and hippopotamuses 

 of the present time, a fact which induced Cuvier to unite them 

 in one order, the ' ' Pachydermata, " a term which has passed 

 into vernacular, if metaphorical, usage. No doubt also, 

 several extinct groups, such as the fAmblypoda and the 

 perissodactyl family of the tTitanotheriidae, would have been 

 included, had they been known in Cuvier's day. In the largest 

 and heaviest of these animals, the elephants, famblypods and 

 ftitanotheres, there are many close correspondences in all 

 parts of the skeleton, which are clearly due to the mechanical 

 necessities imposed by the support of immense weight, and 



