Morphological Variation and Its Causes in A. tigrinum 69 



All that I have said applies to the teeth of larvae like the can- 

 nibals on plate VIII, or several centimeters smaller. I have not 

 been able adequately to follow all the phases of later develop- 

 ment, but some further facts are interesting. Often no further 

 development takes place, but even a retrogression begins. What 

 becomes of these cannibal teeth when the habit is finally quite 

 given up I have not fully determined. I think they are probably 

 shed entirely. In several older cannibalistic individuals, includ- 

 ing the one on plate VII, and the large transition type on plate IX, 

 a curious change has taken place. The heavy downward project- 

 ing ridges or pads that bore these teeth remain very prominent ; 

 but they are quite destitute of teeth, except along their inner 

 borders, where a few very large teeth point almost directly to- 

 ward the throat. Possibly this loss may be connected with a 

 temporary assumption of some other habit of feeding. Some of 

 the specimens had fed, at least partially, upon snails. So far as 

 I have examined adults resulting from cannibals or semicanni- 

 bals, I find the palatine teeth are shed entirely. The ridges which 

 bore them are also reduced, although remaining many times more 

 prominent than in ordinary adults. In larvae which are full can- 

 nibals but which have reached a size somewhat greater than 

 those on plate VIII, there seems to be no further hypertrophy in 

 the actual size of the teeth, but differentiation and adaptation 

 continue in other ways. Thus the anterior brushes come to ex- 

 ceed still more the posterior, rendering these latter sometimes 

 relatively insignificant. And -at the same time they change their 

 angle of projection; the brush comes to face, not directly down- 

 ward, but downward and backward, so that nearly all of the 

 palatine teeth point more or less toward the throat. 



It seems to me very interesting to note these rapid, progressive 

 variations of so strikingly adaptive a nature. And it is still more 

 interesting because the species shows so many other variations 

 which are but partially or not at all adaptive. Contrast for a 

 moment these adaptive variations in form and habit with varia- 

 tions in the time of metamorphosis. It- is true that metamor- 

 phosis has been supposed to be the very quintessence of adapta- 

 tion. But my study of it in this species shows it to be here of the 



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