Zool.— Vol. I.] RITTER—DIEMYCTYLUS TOROSUS. 91 



dependent on " food, season, nor environment." But I have 

 seen nothing to indicate that the wide-tailed, smooth-skinned 

 condition of the males is ever assumed without the adop- 

 tion of the aquatic mode of life; and so obviously and per- 

 fectly is it an adaptation to such a life that one can scarcely 

 believe it to have been produced by any other cause. 



To say that the characters distinguishing it are " second- 

 ary sexual characters," and so dismiss the subject with the 

 supposition that an explanation of the facts has been given, 

 is wholly unsatisfactory, even though the males alone present 

 these characters, and that at the time of sexual activity. 



It appears to me that instead of regarding the seasonal 

 changes as independent of environment, or as having been 

 produced in response to the needs of the reproductive func- 

 tion, we should come nearer to a satisfying explanation by 

 supposing that originally and immediately they were caused 

 by change in the mode of life of the animals; and that 

 now the characters have become so thoroughly established by 

 heredity that they have acquired a considerable degree of 

 independence of the causes which produced them. 



It seems probable that the reason why D. torosus re- 

 verts to the rough-skinned, narrow-tailed state after it has 

 passed a period in the opposite condition, whereas D. viri- 

 descens never leaves the viridescent, aquatic form after hav- 

 ing once assumed it, is to be found in the difference in 

 habitat of the two species. The region inhabited by viri- 

 descens is not particularly drier at one season of the year 

 than another, and the streams and ponds which the animals 

 make their home are perennial; so that, so far as environ- 

 ment is concerned, there is no reason why they should leave 

 the water when once they have betaken themselves to it 

 and become well established in it. With torosus the case is 

 different. Throughout the greater portion of its range very 

 many of the streams and ponds in which it lives during the 

 winter and early summer dry up almost entirely (I doubt 

 considerably if the larvae ever come to metamorphosis in 

 streams that wholly disappear during the summer) ; and as 

 a consequence the animals find it to their advantage, in 



Proc. Cal. Acad. Sci., 3D Ser., Zool., Vol. I. (3) Nov. 18, 1896. 



