Morphological Variation and Its Causes in A. tigrinum 49 



the first conditions of limb-building brought very close to us. I 

 have said that these effects of overnutrition are not to be classed 

 with those of use and disuse as regards transmission. The rea- 

 son is that they result essentially from the single primitive factor 

 of appetite, and, surprising as it may perhaps seem, I have con- 

 siderable evidence that this is hereditary. 



Before leaving the posterior limb I must speak of one more 

 variation which is, in its way, as interesting as any I have men- 

 tioned, but which is, it would seem, almost purely a phenomenon 

 of habit, of adaptive functioning. I refer to the dift'erent man- 

 ners in which the limb is used in locomotion, as well as to the 

 different positions in which it is held when at rest. Even in 

 breeding adults, taken at the same time and place, the contrast 

 is most striking, especially if the animals are placed under suit- 

 able conditions, e. g. in a large vessel with very shallow water 

 which admits of free and natural movements. Some animals ex- 

 tend the posterior limbs, when at rest, straight outward, at right 

 angles to the body and with the soles of the feet nearly or quite 

 vertical. Such positions are partially shown in figure 2, plate 

 IV, and figure i, plate VI, although neither of these shows the 

 foot sufficiently vertical, the effort in the photography having 

 been to show as much of the foot as possible. Such animals as 

 these use the limbs in nearly this position of extension when 

 walking ; in bringing the foot forward, the knee is flexed but lit- 

 tle, and in the backward stroke the limb is used almost like a 

 straight lever, the foot moving through a segment of a circle. In 

 walking on land or overcoming strong resistance the joints are 

 flexed more and the soles are more completely planted on the 

 substratum, but the tendency toward this awkward, straight limb 

 is always manifest. Omitting intermediate types, which of course 

 occur, one occasionally finds large adults which attract attention 

 at once by an opposite use of the limbs and feet. Such specimens 

 stand on their feet, even when at rest in the water, or at least 

 they frequently do so, and the feet are even planted close to the 

 body, all of the joints — hip, knee, and ankle — being very sharply 

 flexed. The knees of such animals are, indeed, thrust upward in 

 a most lizard-like attitude, and the attitude is quite habitual. In 



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