Chapter Ten 



The Tetforal Limb 



IT is A WELL established belief that all terrestrial vertebrates were orig- 

 inally derived from a fish-like aquatic ancestry. The anterior limb, or 

 ichthyopterygium, of fish has certain well defined characteristics, among 

 which is the lack of clear distinction between the proximal elements 

 ("brachium" and "antibrachium"), the ocurrence usually of more than 

 five series of distal components ("digits"), and the indeterminate num- 

 ber of elements ("phalanges") of which each of these are composed, ac- 

 cording to the exigencies of individual cases. In mammals the anterior 

 limb, or cheiropterygium, is composed of distinct brachial and anti- 

 brachial segments, the digits never number more than five, nor the 

 phalangeal ones (including metatarsi) more than four except in whales. 

 What is more logical than to presume that some of the steps taken by 

 the mammalian stock as it arose, by whatever process and by whatever 

 path, from a fish-like ancestry, should eventually be retraced in more or 

 less complete degree as some of its representatives once more become 

 completely fitted for an aquatic existence ? Working with such a thesis 

 as a tentative basis certain of the trends which are found to be exhibited 

 take on added significance. 



When a mammal first takes to the water the fore limb is usually, if 

 not always, used as an aid to locomotion, and there will be a lengthy pe- 

 riod during which its function is very inefficient for the reason that it 

 is ill fitted for the part that it plays. The final fate of the fore limb 

 in respect to aquatic modifications undoubtedly depends upon a great 

 number of factors, but it seems that the chief determinant rests upon 

 the question of whether or not the hind limbs or (and) the tail gain 

 evolutional ascendancy over the pectoral appendages. The chances of 

 the hind limbs gaining the lead are much more than even. In the first 

 place, with very few exceptions (some sorts of bovines, as Bison, the 

 hyenas, etc.) the hind limbs of terrestrial mammals are larger, more pow- 

 erful, or both, than the complementary pair. They would then be more 

 vigorously kicked, and would gradually play an increasingly important 

 part in aquatic locomotion. Second, the fore limbs may be used for 

 other purposes besides propulsion when the animal is in the water, as 



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