March 20, 1879] 



NATURE. 



463 



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 mathematician Del- 

 boeuf, may be here applied ; and I will quote Mr. 

 Murphy's condensed statement (" Habit and Intelli- 

 gence," 1879, P- 241) with respect to it: "If in any 

 species a number of individuals, bearing a ratio not infi- 

 nitely small to the entire 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 supposed to be atavism to a 

 very remote progenitor, and this may have wholly pre- 

 vailed over any tendency to atavism to more recent pro- 

 genitors ; and of such prevalence analogous instances 

 could be given. Charles Darwin 



Blumenaii, St. Catharina, Brazil, 

 January 21, 1879 

 Mv 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 Bromeliae. Lately I found, in a large Bromelia, a 

 little frog {Hy lodes ?), bearing its eggs on the back. The 

 eggs were ver}' 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 fruit- 

 less trials a tolerable photograph could be obtained). The 

 tadpoles, on emerging from the eggs, were already pro- 

 vided with hind-legs ; and one of them lived with me about 

 a fortnight, when the fore-legs also had made their appear- 

 ance. During this time I saw no external branchiae, nor 

 did I find any opening which might lead to internal 

 branchiae. 



Fig. I. 



There is here another locality in wliich a peculiar fauna 

 lives, viz., the rocks of waterfalls, which are of very fre- 

 quent occurrence in almost all our mountain rivulets. On 

 these rocks, along which the water is slowly trickling 

 down, or which 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 HydropsychidcB, 

 hydroptilidcE, and Sericostomatidce [flelicopsyche]), as well 

 as those living in the Bromeliae (a species belonging to the 

 LeptoceridcE), are distinguished by a very interesting fea- 

 ture. 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 pupce 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 exa- 

 mined, 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 {fleli- 

 copsyche) the fringes of the 

 legs, used for swimming, are 

 well developed. 



This abortion of the useless 

 fringes in the caddis-flies in- 

 habiting the Bromeliae and 

 waterfalls appears to me to 

 be of considerable interest, 

 because it carmot be con- 

 sidered, as in many other 

 cases, as a direct consequence 

 of disuse ; for at the time 

 when the pupae leave their 

 cases and when the fringes of 

 their feet are proving either 

 useful or useless, these fringes 

 as well as the whole skin of 

 the pupa, ready to be shed, 

 have no connection whatever 

 with the body of the insect ; 

 it is therefore impossible 

 that the circumstance of the 

 fringes being used or not for 

 swimming, should have any 

 influence on their being de- 

 veloped or not developed in 

 the descendants of these in- 

 sects. As far as I can see, 

 the fringes, though useless, 

 would do no harm to the spe- 

 cies, in which they have dis- 

 appeared, and the material 

 saved by their not being 

 developed appears to be 

 quite insignificant, so that 

 natural selection can hardly 

 have come into play in this 

 case. The fringes might disappear casually in some indi- 

 viduals ; but, without selection, this casual variation would 

 have no chance to prevail. There must be some constant 

 cause leading to this rapid abortion of the fringes on the 

 feet of the pupae in all those species in which they have 

 become useless, and I think this may be atavism. For 

 caddis-flies, no doubt, are descended from ancestors which 

 did not live in the water, and the pupae of which had no 

 fringes on their feet. Thus there may even now exist 

 in all caddis-flies an ancestral tendency to the production 

 of hairless feet in the pupae, which tendency in the common 

 species is victoriously counteracted by natural selection, 

 for any pupa, unable to swim, would be mercilessly 

 drowned. But as soon as swimming is not required and 

 the fringes consequently become useless, this ancestral 

 tendency, not counterbalanced by natural selection, vn\t 

 prevail, and lead to the abortion of the fringes. 



I do not remember having seen, in any list of cleisto- 

 gamic plants, the Podostemaceae. These curious little 

 aquatic plants, which Lindley placed near the Piperaceae, 

 Kunth between the Juacagineae and Alismaceae, and which 

 Sachs considers as being of quite dubious affinity, cover 

 densely the stones in the rapids of our rivers ; on the 

 branches which come above the surface of the water, 

 there are pedunculated, open, fertile flowers ; but there 

 are numerous sessile flower-buds also on the branches. 



Fig. 2. Fig. 3. 



Fig. 2. — Tibia and tarsus of the 

 two pairs of legs of the pupa of 

 a specleso" Leptocaridae, iahabit- 

 iaz BroiasUae. Fig. 3 — The 

 sane of a nearly allied species 

 inhabiting rimlets. 



