Morphological Variation and Its Causes in A. tigrinum 13 



long-bodied, and long-tailed animal becomes well-nigh perforce 

 an undulatory swimmer ; while the strong-limbed, short-tailed, 

 heavy-bodied specimen, when these characteristics are rapidly 

 forced upon it, is, under certain circumstances, just as forcibly 

 induced to become a crawler. In such instances, where the devel- 

 opment plainly at first conditions the action, the cause is again 

 purely nutritive. I will mention a single instance of two larvae 

 which in the course of a few weeks assumed such different modes 

 of locomotion that, with their equally contrasting appearances, 

 they never failed to call out a burst of laughter when displayed to 

 a laboratory visitor. In my experiments upon metamorphosis I 

 placed, at intervals of a few weeks, a number of larvae, under 10 

 cm. in length, in a flat pan, two feet in diameter, with but an inch 

 of water, and the whole so tilted that the larvae were well out of 

 water on the shallow side although covered at the deeper side. 

 Most larvae, though not all, are checked in their feeding for a 

 time by this treatment, and some become very light eaters. Such 

 animals soon take on a very slender form, and are as rounded and 

 graceful in outline as they are slender. In the experiment alluded 

 to one larva especially became thus modified, growing slowly but 

 steadily, and retaining its larval condition longer than any other 

 fed larva among the hundreds of my laboratory specimens. It 

 resembled figure 4, plate II, save that the head was still narrower 

 and the general outlines more slender and evenly cylindric, with 

 gills and limbs still shorter. Despite the shallow' water which, 

 according to all common theories, should have stimulated the ani- 

 mal to a systematic use of its limbs, it used these, members less 

 and less, until finally the posterior limbs were never used at all, 

 the soles remaining vertical when the animal was at rest, and the 

 whole limb being appressed to the side the moment that locomo- 

 tion began. And very regularly did this individual take its exer- 

 cise, circling round and round its narrow quarters, vigorously 

 ascending the shallow side by means of strong serpentine move- 

 ments. So striking were these movements and so much did they 

 exaggerate the effect of the extremely slender form that one 

 seemed to see in this animal a variation which might indeed lead 

 to a Siren or an Amphiuma. 



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