CAUSES OF GENETIC VARIATION 231 



Such evidence could be used for what it is worth ; but pending an 

 inquiry of this kind I am disposed to regard these observations of 

 variation following on parental injury as suggestive rather than 

 convincing. 



Some evidence of a remarkably interesting kind has been 

 collected by J. H. Powers 19 respecting the structure and habits 

 of Amblystoma tigrinum, which led him to the conclusion that 

 striking differences in the form, anatomy, and developmental 

 processes could be effected directly by change in the conditions 

 of life. It is well known that a profusion of forms, distinct in 

 various degrees, is grouped round Amblystoma tigrinum. Some 

 of these are believed to be geographically isolated, others occur 

 together in the same waters, and, as usual, authorities have dif- 

 fered greatly as to the number of names to be given. These forms 

 were studied in detail by Cope who described them in the Ba- 

 trachia of North America. The view which he inclined to take 

 was that the individual variations of Amblystoma tigrinum re- 

 sulted from variations in the time and completeness of the 

 metamorphosis, and these were regarded as due to external 

 causes, such as differences in season, temperature, and geo- 

 graphical conditions. Powers, however, states that collecting 

 within a radius of six or eight miles he found almost if not quite 

 the whole "gamut of recorded variation in this species." Some, 

 however, as he states, occurred rarely except under experimental 

 conditions, but considerable differences in temperature were not 

 found necessary in producing them. Every year, he says, he 

 has been able to add to the number of peculiar types found in 

 the same small area in nature, until the amount of natural 

 variation at least equals that seen by Cope in the collections of 

 the National Museum and those of the Philadelphia Academy. 



Powers states that his observations by no means confirm 

 Cope's view that these differences are in the main referable to 

 variation in the completeness of metamorphosis, and on the 

 contrary, he regards metamorphosis as on the whole a levelling 

 process, tending to obliterate diversity. The enormous dif- 



19 J. H. Powers, "Morphological Variation and its Causes in Amblystoma 

 tigrinum." Studies from the Zoological Laboratory. The University of Nebraska, 

 No. 71, 1907. 



