Morphological Variation and Its Causes in A. tigrinum 41 



With these may be compared also figure 6, plate V. Figure 5 is 

 typical ; figure 4 shows the character in an unusual degree, and yet 

 it had, in this case, been wholly assumed in one summer's heavy 

 feeding, after a period of severe starvation out of the water. 

 Figure 6, plate V, shows but a trace of the peculiarity, owing to 

 the unusual roundness of the head, and yet the position of the 

 nostrils is still marked by slight angular projections. Figure 5, 

 plate V, shows the head of the adult figured in profile just above 

 it, figure 3, plate V. In this instance the very marked truncation 

 is the result of the assumption of the character by the larva, and 

 later of its reaccentuation by the aquatic life of the adult after 

 metamorphosis. 



VARIATIONS IN THE POSTERIOR LIMBS 



The posterior limbs are much more variable than are the an- 

 terior, and I have, in the main, confined my attention to them. 

 Moreover, I shall limit myself still further to certain special vari- 

 ations which I have studied. Viz.; first, to the robustness or slen- 

 derness of the limb; second, to the habitual posture and manner 

 of use of the limb ; and third, to the foot and toes, their roundness 

 or depression with bordering skin-folds, and lastly their basal 

 webbing. I may in passing also mention the variations of the 

 posterior limb in length ; they are very great and but partially 

 correlated with other bodily features. Such observations as I 

 have made indicate that their rationale will also be found in dif- 

 ferent rates of growth together with different modes of function. 

 But my observations as yet lack completeness and definiteness 

 upon this point. 



On the matter of the robustness of the limb, however, I have 

 made sufficient observations to reach, I think, conclusive results. 

 The amount of variation in it, too, is very great. Compare figures 

 6 and 7, on plate IV, with figure 9 of the same plate. The first 

 two figures show but parts of the limb, and the view is not the 

 same as in the 'last. But a glance shows that these limbs as well 

 as the feet differ prodigiously in the degree of their massiveness. 

 Yet this difference is due neither to age, size, nor sex. All are 



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