THE PELVIC LIMB 



Enlargement of the hind feet frequently results from a life in the water 

 but this character may develop slowly and one must be careful always 

 to compare forms in which approximately the same method of swimming 

 is employed. It would be entirely without significance to compare in 

 this respect the feet of hippopotamus, seal, otter and beaver, because 

 the stimulus for foot development is different in each. In comparisons 

 it must also be remembered that various terrestrial activities tend to in- 

 crease the size of the hind feet. Thus the percentage of the hind foot 

 length to head and body length may approach 30 per cent in arboreal 

 squirrels. Probably for the reason that it is slightly given to arboreal 

 activity this percentage in the wood rat (Neotoma) may be greater than 

 20. But in several terrestrial shrews of different genera the above per- 

 centage varies around 16.5, in various terrestrial rodents, such as the 

 common brown rat (Rattus norvegicus) it is about 17.5, although in the 

 same species it may vary from 16 to 19. In various other non-aquatic 

 shrews, however, such as Sorex pact fie us, the foot may be from 19 to 

 23 per cent of the head and body, but this is probably because of some 

 definite stimulus of which we do not know. On the whole it seems 

 safe to say that this percentage in most terrestrial insectivores of the 

 shrew type, and of rodents, will usually be found to be less than 20. 

 Even in a rodent with definitely aquatic propensities it may fall below 

 this figure (as little as 17.5 in Arvicola) . Selecting a series of aquatic 

 forms that chance to be available it is found that in the slightly aquatic 

 genus Dasymys, without other modification of the feet, this percentage 

 is 19 to 21, in Neomys foidens about 21.5, in Atophyrax (Sorex ben- 

 dirii) 22.5 Ichthyomys (one specimen) 23, Hydromys 22.5 to 25, 

 Galemys 26 to 30, Ondatra 38.5, and Castor 38 to 40. These figures are 

 only approximately correct as they may be based on but a single specimen 

 and the collector's measurements may not be accurate. They do show, 

 however, that in such small mammals as swim chiefly by means of the 

 hind feet there is a tendency for the elongation of these members, in 

 degree according to aquatic modifications in other respects. Such elonga- 

 tion may be barely appreciable until after the toes have become webbed 

 or fringed, and may finally result in a foot that is twice as long as in 

 a more generalized, terrestrial genus. There is frequently an appreciable 

 lengthening of the toes in relation to the foot proper, and a more marked 

 spreading of these members so as to present a greater surface (by means 

 of webbing or fringing) to the water. 



Another modification which is often encountered in aquatic rodents 

 and insectivores is, according to the spirit specimens that have chanced 



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