Morphological J'anatioii and Its Causes in A. tigrinuni 3 



The inadequacy of these general assumptions first became evi- 

 dent when, within the narrow radius of six or eight miles, I began 

 to collect many of Cope's most important geographical types. At 

 the present time, indeed, judging as best I may from Cope's writ- 

 ten descriptions, it would seem that I have found nearly if not the 

 entire gamut of recorded variation in this species. Some of these 

 varieties have occurred rarely except under experimental con- 

 ditions. But considerable differences in temperature have not been 

 found necessary in producing them, and continued search within 

 the same narrow area has added yearly to the number of peculiar 

 types found in nature, until the amount of natural variation 

 equals, I think, if it does not exceed that recorded by Cope from 

 the extensive collections of the National Museum and the museum 

 of the Philadelphia Academy. Varying geographical habitat is, 

 then, not a necessary factor in the production of these variations. 



As to the persistence of larval characters through partial, inter- 

 rupted, or asynchronous metamorphosis, I find this, although not 

 correlated strongly with differences of temperature as a cause, to 

 be a real, though only a minor, factor of variability. Variations in 

 the development of the tongue, which may yield adults in every 

 condition, from those with thick tongues filling nearly the whole 

 floor of the mouth to those which show (rarely) quite tongueless 

 larval mouths, are plainly of the nature which Cope assigns. And 

 in the case of this character more than of any other I have had 

 instances in which a retarded development seemed to be due to 

 metamorphosis at low temperatures. Curiously, too, other 

 instances have occurred of fine adults without tongue develop- 

 ment in which the retardation was due to the retardation and 

 interruption of metamorphosis by high feeding at high tempera- 

 tures (30° C). Variations in the character and the curve of the 

 palatal teeth, which I may say in passing are much greater than 

 recorded by Cope, are also in part, though in part only, inter- 

 pretable in terms of departure from or retention of the typical 

 larval arch of teeth. 



But, aside from these features which do conform in part to the 

 principles laid down by Cope, I find little else among the whole 

 range of the variations of the species in harmony with them ; or 



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