On a frog having eggs on its back. 



On the abortion of the hairs on the legs of certain 



coddis-flies (Phryganiden) 1 ). 



(Letter to Ch. Darwin.) 

 Mit 3 Textfiguren. 



Several of the facts given in the following letter from Fritz Miiller, especially 

 those in the third paragraph, appear to me very interesting. Many persons have 

 felt much perplexed about the steps or means by which structures rendered useless 

 under changed conditions of life, at first become reduced, and finally quite dis- 

 appear. A more striking case of such disappearance has never been published 

 than that here given by Fritz Miiller. Several years ago some valuable letters 

 on this subject by Mr. Romanes (together with one by me) were inserted in the 

 columns of Nature. Since then various facts have often led me to speculate on 

 the existence of some inherent tendency in every part of every organism to be 

 gradually reduced and to disappear, unless in some manner prevented. But beyond 

 this vague speculation I could never clearly see my way. As far, therefore, as I 

 can judge, the explanation suggested by Fritz Mtiller well deserves the careful 

 consideration of all those who are interested on such points, and may prove of 

 widely extended application. Hardly anyone who has considered such cases as 

 those of the stripes which occasionally appear on the legs and even bodies of 

 horses and apes or of the development of certain muscles in man which are not 

 proper to him, but are common in the Quadrumana - - or again, of some peloric 

 flowers - will doubt that characters lost for an almost endless number of gene- 

 rations, may suddenly reappear. In the case of natural species we are so much 

 accustomed to apply the term reversion or atavism to the reappearance of a lost 

 part that we are liable to forget that its disappearance may be equally due to this 

 same cause. 



As every modification, whether or not due to reversion, may be considered 

 as a case of variation, the important law or conclusion arrived at by the mathe- 

 matician Delbceuf , may be here applied ; and I will quote Mr. Murphy's condensed 

 statement ("Habit and Intelligence", 1879, p. 241) with respect to it: "If in any 

 species a number of individuals, bearing a ratio not infinitely small to the entire 



i) Nature 1879. Vol. XIX. p. 462464. 



