On the abortion of the hairs on the legs of certain coddis-flies. 



number of births, are in every generation born with any particular variation which 

 is neither beneficial nor injurious to its possessors, and if the effect of the variation 

 is not counteracted by reversion, the proportion of the new variety to the original 

 form will constantly increase until it approaches indefinitely near to equality." 

 Now in the case advanced by Fritz Miiller the cause of the variation is sup- 

 posed to be atavism to a very remote progenitor, and this may have wholly pre- 

 vailed over any tendency to atavism to more recent progenitors; and of such 

 prevalence analogous instances could be given. Charles Darwin. 



My Dear Sir, 



If I remember well, I have already told you of the curious fauna which is 

 to be met with between the leaves of our Bromeliee. Lately I found, in a large 

 Bromelia, a little frog (Hylodes?}, bearing its eggs on the back. The eggs were 



very large, so that nine of them covered the whole back 

 from the shoulders to the hind end, as you will see on the 

 photograph accompanying this letter, Fig. i (the little animal 

 was so restless that only after many fruitless trials a toler- 

 able photograph could be obtained). The tadpoles, on emer- 

 ging from the eggs, were already provided with hind-legs; 

 and one of them lived with me about a fortnight, when the 

 fore legs also had made their appearance. During this time 

 I saw no external branchiae, nor did I find any opening which 

 might lead to internal branchiae. 

 F - r There is here another locality in which a peculiar fauna 



lives, viz., the rocks of waterfalls, which are of very frequent 



occurrence in almost all our mountain rivulets. On these rocks, along which 

 the water is slowly trickling down , or \vhich are continually wetted by the 

 spray of the waterfall, there live various beetles not to be met with anywhere 

 else, larvae of diptera and caddis-flies, and a tadpole remarkable for its unusually 

 long tail. 



The pupae of caddis-flies living on the rocks of waterfalls (I examined three 

 species belonging to the Hydropsychidce, Hydroptilidcc and Sericostomidce [Helico- 

 psyche]), as well as those living in the Bromeliae (a species belonging to the 

 Leptoceridce), are distinguished by a very interesting feature. In other caddis- 

 flies the feet of the second pair of legs (and in some species those of the first 

 pair also) are fringed in the pupae with long hairs, which serve the pupa, after 

 leaving its case, to swim to the surface of the water for its final transformation. 

 Now neither on the surface of bare or moss-covered rocks, nor in the narrow 

 space between the leaves of Bromeliae, the pupae have any necessity, nor would 

 even be able, to swim, and in the four species living on such localities which I 

 examined, and which belong to as many different families, the feet of the pupae 

 are quite hairless, or nearly so, while in allied species of the same families or 

 even genera (Helicopsyche) the fringes of the legs, used for swimming, are well 

 developed. 



This abortion of the useless fringes in the caddis-flies inhabiting the Bromeliee 

 and waterfalls appears to me to be of considerable interest, because it cannot be 



