Morphological Variation and Its Canscs in A. tigrinuni 2>3 



very abundant. The general small size of the head and of the 

 gape of the mouth is thus readily explained by these conditions ; 

 but the high arch of the profile is not obviously so explained, and 

 I am forced in regard to it partially into conjecture. A partial 

 explanation lies in the fact that whatever tends to broaden the 

 head also tends to flatten it, to reduce its height. These heads 

 were not flattened by any special influence, and consequently as- 

 sumed a form that, in cross-section, more nearly bears out the 

 lines of the body, which is cylindrical. However, it is probable 

 that a special method of feeding actually tended to shorten the 

 head and to arch the profiles. As I watched the animals when 

 the reservoir was clear they were almost continually seeking food 

 on the bottom. To do this the animal flexes the head- sharply 

 downward (the very opposite of the flexure when the animal is 

 seining, so to speak, for daphnids), roots with the muzzle among 

 the ooze at the bottom, and seizes by an occasional jab anything 

 that is seen or felt to move. This method of feeding is extremely 

 natural and is resorted to by the species whenever there are no 

 free-swimming organisms to draw the attention upward. Under 

 such circumstances the mouth is used but seldom and is not 

 widely opened ; the downward flexure of the head, sharp as one 

 can flex the first joint of an extended finger, is the dominant fea- 

 ture of the process, and it would seem that this might have some- 

 thing to do with the development of these very convex profiles. 



As already indicated, ^these high-profiled, but small-headed 

 adults simulate and even exaggerate Cope's "Eastern variety." 

 This he thought resulted from the fuller development and more 

 nearly complete metaniorphosis due to a warmer climate (New 

 Jersey) ; whereas the ''Western form," with broader, flatter head, 

 was an instance of less nearly complete metamorphosis or "larval 

 tendency" resulting from a colder climate (Minnesota). The 

 specimens just discussed illustrate w^ell the inadequacy of Cope's 

 assumptions. They exaggerate the Eastern form ; yet they grew 

 as larvae in the relatively cool water of nearly eight feet in depth. 

 As to metamorphosis, I placed a dozen of the larvae (several of 

 them already sexually mature), in a dark, cold basement, or in a 

 very large, cool cistern. Either of these situations ought to equal 



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