54 The Rodent ia in Evolution. 



iavorable variation which could be seized upon by natural 

 selection and become adaptive modifications of structure^ 

 announced it as his belief from the study chiefly of verte- 

 brate osteology, that the uses of the organs were productive 

 of adaptive shapes and construction, so that as a result of 

 change of habit a creature, though by inheritance from its 

 parents it would have the family likeness, would also have a 

 certain unlikeness, leading toward a perfect fitness for a new 

 habit. This change, he argued, would be seized by inherit- 

 ance and reappear in the next generation, when it would be 

 improved upon, and so on, extending to the organization 

 more and more deeply till a new organization would result. 

 It would be necessary to such a mechanism of evolution 

 that variations due to use should be shown to be transmis- 

 sible by inheritance. The Post-Darwinians, represented by 

 Professor Lankester, among the English, claim that inherit- 

 ance does not extend to these acquired variations, but that 

 variations which appear at birth in the animal are inherit- 

 able. When the extreme isolated cases are searched over, 

 an abundance of examples can be produced which seem to 

 substantiate Cope's position, but when the data are all 

 carefully surveyed, the discovery of animals with special 

 habits, but not correlated structure, are not easily explained. 

 They seem to be creatures waiting for evolution to come ta 

 their aid, and by giving them specialized structures to help 

 them in the struggle for life. If use develops function, animals 

 as habitually fossOrial as the striped gopher ought, it would 

 seem, to be so in structure far more completely than they 

 are. So the muskrat ought to have a more perfectly webbed 

 foot, and so on. The Neolamarkians always answer such 

 objections by the assumption that time has not yet elapsed 

 for the changes to be brought about. But to me the detailed 

 study of the rodents does not appear to favor Neolamark- 

 ism. Though it so plainly indicates that evolution has 

 taken place, it also plainly indicates that structure is ex- 

 tremely conservative, and does not readily lend itself to 

 change. We do not yet know how long the rodents have 

 been as they are to-day, but the main line of descent had 

 diverged by Tertiary time. So far as data are at hand from 

 which to calculate the rate of evolution, the rate seems 



