THE PECTORAL LIMB 



ments. Without a tail of fair proportions or with a body of unusual 

 width (hippopotamus) the dog-fashion method of swimming will long 

 be followed, the important function of the fore limb then being largely 

 as an antagonist, to neutralize the oblique, sidewise impulse furnished by 

 the separate kick of each hind foot. In such case the manus might 

 be expected to acquire aquatic modification almost if not quite as rapidly 

 as the pes. But still, each sort of mammal is a law unto itself, and 

 accordingly it is necessary that each be considered separately, with some 

 unavoidable repetition of facts. 



In the majority of insectivores and rodents the manus is totally un- 

 modified. They appear to have no need for especial speed in the water, 

 the food being mainly herbivorous or insectivorous, and the feet are 

 presumably folded against the body so as to offer no resistance while 

 swimming. They can have no use for propulsion and in the water 

 can act only as grasping organs for securing prey. Manual activity will 

 thus be in direct proportion to the amount of terrestrial locomotion 

 indulged in, and as the latter decreases one would expect that a tendency 

 for reduction of the limb would increase. 



Concerning various sorts of slightly aquatic mammals the statement 

 is frequently made that the fore feet are partially webbed, when an 

 examination of the specimens fails to indicate the fact. Apparently 

 some authors have been prone to interpret the membranes stretching 

 between the base of the toes as aquatically-adapted webbing when as a 

 matter of fact this is no better developed than in a great number of 

 comparable forms with habits that are exclusively terrestrial. Undoubt- 

 edly in the desman, Australian water rats and some otters the webbing 

 of the anterior appendage is slightly better defined than what may be 

 considered as normal, but it is of very slight degree indeed and may 

 be partly illusory because there is also a slight tendency for a lessened 

 definition of the digits following decreased use of the limb. In Chimar- 

 rogale, however, there is more of a hairy fringe to the manus than ter- 

 restrial insectivores show, this matching a similar but far stronger de- 

 velopment upon the hind feet. 



Here a question intrudes itself that should be accorded brief mention. 

 Irrespective of the fact that hypertrophy of the posterior limbs is often 

 accompanied by atrophy of the anterior ones (compensational develop- 

 ment) , is there not a tendency, however slight, for the manus to adopt 

 the same quality of equipment as that occurring upon the pes: the same 

 quality of nails, webbing, or hairy fringes? Needless to say, // there 



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