52 The Rodent ia in Evolution. 



figures 1 and 2. These, on examination, are not found to 

 present anything in common beyond the mammalian and 

 rodent construction found in all the class and order. In one 

 the narrow frontal and parietal, in the other these bones are ■ 

 broad; in one the very short molar, in the other a long 

 and stout one reaching from the squamosal to the lachrymal 

 bones. In the bones of the hind leg we find the anchylosed 

 fibula of the muskrat, the free fibula of the beaver, but we 

 do find in each a strong ridge on the hinder side of the 

 tibia (as also in many other rodents). On the other hand if 

 these be compared with animals of very unlike habits we 

 shall find many points of resemblance. The beaver and the 

 ground squirrel can be compared, figs. 2 and 5, and the 

 muskrat and the brown rat, figs. 1 and 3, and important 

 similarities at once come to light. Upon Professor Agassiz's 

 view this would mean that when aquatic animals were 

 being produced several plans used also in producing terres- 

 trial and fossorial animals were adapted to the purpose in 

 spite of the fact that one must have been more suitable than 

 others and that perhaps some wholly non-rodent plan 

 would have been more suitable still. Webbed feet are better 

 for swimming than those that are not webbed; the beaver 

 has good webbed feet and the 'muskrat has feet that are 

 hardly webbed at all. It is quite obvious that the implica- 

 tions of the multitudes of such facts as these would be if 

 they referred to the work of an artisan, that many poor 

 models and few good ones were available when he w^as at 

 work making his productions. 



If, however, the animals of today be regarded in the 

 light of evolution, all these matters at once become clear. 

 Fossorial animals, for instance, vary greatly in the degree 

 to which they are specialized for burrowing. Some are 

 burrowers in habit, but hardly, if at all, in structure. 

 Others have external but not the more deeply seated 

 specialization of structure, while still others are completely 

 fossorial in structure as well as habit. The fossorial habit 

 in its external form, as in the moles, is marked by the 

 following features, short hair, no external ear, eyes abortive 

 and subdermal, anterior part of body stronger than posterior 

 fore limbs short, stout hand, broad stout nails long, tail 



