Morphological Variation and Its Causes in A. tigrinuni 43 



'"aquatic" feet of the most extreme type, the toes being much 

 depressed and heavily bordered with folds of skin quite to the 

 tips. On the other hand, a score of large adults that had lingered 

 unusually long after the breeding season — until the 9th of May — 

 in an extra good feeding ground under six to eight feet of water, 

 showed feet of diverse but perfectly usual types, broad toes and 

 narrow toes without sign of adaptive modification. The habits, 

 too, of these animals when in the water are not such as to lead to 

 the development of a swimming foot. At the breeding season 

 the males are nervously active in the water, courting each other 

 as well as the females, and performing astonishing feats of jug- 

 glery, carrying a female — or a friendly male who consents for 

 the nonce to the substitution — for yards upon the upraised tip of 

 the snout. But in these movements, which I have watched for 

 hours, they remain almost exclusively upon the bottom, undulat- 

 ing the long tail and even the body from side to side, the feet 

 meanwhile making enormous strides, touching the bottom by the 

 tips of the toes only. The movements of food-seeking, etc., which 

 often alternate with those of courtship, are still more of the na- 

 ture of walking, unless swarms of daphnids are present nearer 

 the, surf ace; then adults may frequently be seen swimming slowly 

 with an unsteady sidewise rolling motion. The feet, however, 

 play almost no part in the propulsion. And when the animals do 

 swim vigorously, as in rising out of deep water for air, the limbs 

 are pressed tightly to the sides. On the whole, similar statements 

 may be made for the larvae, although the larva seems to me to 

 use the foot more than does the adult in swimming ; its use is, 

 however, mainly confined to making sudden turns to the right or 

 left. Very marked variations in this respect also occur, and in 

 the larvae certainly to a greater extent than in the adult. 



What, then, is the cause of these varying types of feet, which 

 at first sight do suggest so irresistibly the idea of adaptations to 

 diflfering degrees of aquatic life? I regret that I must again an- 

 swer so nearly in the vein in which I have spoken before. But 

 these different types of feet, especially the broad or narrow toes, 

 are again nutrition phenomena. They are developments, in the 

 main, during the life of the larva, not of the adult ; and they result, 



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