18701882] WEISMANN 357 



two sexes of some few birds are there differently modified Letter 268 

 by climate, and I have introduced this fact in the last 

 edition of my Descent of Man. 1 I am, therefore, fully 

 prepared to admit the justness of your criticism on sexual 

 selection of lepidoptera ; but considering the display of 

 their beauty, I am not yet inclined to think that I am 

 altogether in error. 



What you say about reversion 2 being excited by 

 various causes, agrees with what I concluded with respect 

 to the remarkable effects of crossing two breeds : namely, 

 that anything which disturbs the constitution leads to 

 reversion, or, as I put the case under my hypothesis of 

 pangenesis, gives a good chance of latent gemrnules 

 developing. Your essay, in my opinion, is an admirable 

 one, and I thank you for the interest which it has 

 afforded me. 



P.S. I find that there are several points, which 

 I have forgotten. Mr. Jenner Weir has not published 

 anything more about caterpillars, but I have written to 

 him, asking him whether he has tried any more experiments, 

 and will keep back this letter till I receive his answer. 

 Mr. Riley of the United States supports Mr. Weir, and 

 you will find reference to him and other papers at p. 426 

 of the new and much-corrected edit, of my Descent of 

 Man. As I have a duplicate copy of Vol. I. (I believe 

 Vol. II. is not yet published in german) I send it to 

 you by this post. Mr. Belt, in his travels in Nicaragua, 

 gives several striking cases of conspicuously coloured 

 animals (but not caterpillars) which are distasteful to birds 

 of prey : he is an excellent observer, and his book, T/ie 

 Naturalist in Nicaragua, very interesting. 



1 Descent of Alan, Ed. II. (in one volume), p. 423. Allen showed 

 that many species of birds are more strongly coloured in the south of 

 the United States, and that sometimes one sex is more affected than 

 the other. It is this last point that bears on Weismann's remarks 

 (loc. ctf., pp. 44, 45) on Pieris napi. The males of the alpine-boreal 

 form bryoni(u hardly differ from those of the German form (var. 

 vernalis\ while the females are strikingly different. Thus the 

 character of secondary sexual differences is determined by climate. 



For instance, the fact that reversion to the primary winter-form 

 may be produced by the disturbing effect of high temperature (p. 7). 



