Morphological J^ariation and Its Causes in A. tigrinum 5 



many of the special, acquired characters of the more plastic larva 

 (which is, however, in this species, so much like an adult animal) 

 are lost. The resulting adult reverts back, as it were, to the com- 

 moner characters of the specific type. Without stopping to defend 

 my comparison of metamorphosis to reproduction, I will say that 

 it may be taken as nothing more than an emphasis upon the fact 

 which I have so often observed, viz., that an unusual, eccentric 

 larva is transformed during metamorphosis into a less unusual, 

 less eccentric adult. Metamorphosis is primarily a leveling 

 process, accordingly, and in much lesser degree a process product- 

 ive of variation. 



Nevertheless, although metamorphosis obliterates many varia- 

 tions of the larva in part or in whole, it fails to obliterate many 

 in part or in whole. This brings me to the core of my conclusion, 

 which, for clearness sake, I will state here at the outset : the vari- 

 ations of A. tigrinum are acquired variations of the larva. They 

 are due, not, as Cope thought, to partial metamorphosis failing to 

 obliterate typical larval characters, but to the fact that complete 

 and perfectly normal metamorphosis obliterates but a portion of 

 the many acquired characters of the larvae. The study of varia- 

 tion in this species thus becomes chiefly the study of the larva ; 

 and the study of the causes of variation becomes the study of the 

 special conditions of larval life as well as the special life activities 

 which bring about the variations. 



Without attempting completeness, and avoiding, in the main, 

 minor characters, I will discuss the following points : first, varia- 

 tions in elongation of the general body form, or the ratio of 

 transverse to longitudinal dimensions, including in this, in a gen- 

 eral way, both. the head and the tail; second, variations and causes 

 of variation in the special form of the tail ; third, some of the many 

 marked alterations in the shape of the head ; fourth, variations in 

 the posterior feet and limbs. Finally, I shall discuss at some 

 length the remarkable modifications in many organs of the body 

 which result from the special habit of cannibalism. This last wall 

 constitute the most striking instance of acquired characters w'hich 

 I have to record, and, indeed, one of the most striking, I think, 

 which has ever been recorded as occurring in a natural species. 



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