page] 462

FRITZ MÜLLER ON A FROG HAVING EGGS ON ITS BACK—ON THE ABORTION OF THE HAIRS ON THE LEGS OF CERTAIN CADDIS-FLIES, &c.

SEVERAL of the facts given in the following letter from Fritz Müller, 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 disappear. A more striking case of such disappearance has never been published than that here given by Fritz Müller. Several years ago some valuable letters on this subject by Mr. Romanes (together with one by me) were inserted in the columns of NATURE.¹ 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 Müller 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 Quadrumana²—or again, of some peloric flowers—will doubt that characters lost for an almost endless number of generations, may suddenly reappear. In the case of

¹ George John Romanes (1848-1894), Canadian-born zoologist and psychologist who worked at University College London and Oxford. He was one of the youngest and most important of Darwin's scientific colleagues and friends. Romanes 1874a and 1874b and Darwin 1873.

² Quadrumana = primates.

[page] 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 Delbœuf,¹ may be here applied; and I will quote Mr. Murphy's condensed statement ("Habit and Intelligence," 1879, p. 241)² with respect to it: "If in any species a number of individuals, bearing a ratio not infinitely 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 Müller the cause of the variation is supposed to be atavism to a very remote progenitor, and this may have wholly prevailed over any tendency to atavism to more recent progenitors; and of such prevalence analogous instances could be given.

CHARLES DARWIN

 

Blumenau, St. Catharina, Brazil,
January 21, 1879

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 Bromeliae. 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. 1 (the little animal was so restless that only after many fruitless trials a tolerable photograph could be obtained). The tadpoles, on emerging 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.

Fig. 1.

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 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 Hydropsychidae, hydroptilidae, and Sericostomatidae [Helicopsyche]), as well as those living in the Bromeliae (a species belonging to the Leptoceridae), are distinguished by a very interesting feature. In other caddis-flies the feet of the second pairs of legs (and in some species those of the first pair also) are fringed in the pups 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 pupa 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.

Fig. 2      Fig. 3.

Fig. 2.—Tibia and tarsus of the two pairs of legs of the pupa of a species of Leptoceridae, inhabiting Bromeliae.
Fig. 3 —The same of a nearly allied species inhabiting rivulets.

This abortion of the useless fringes in the caddis-flies inhabiting the Bromeliae and waterfalls appears to me to be of considerable interest, because it cannot be considered, 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 developed or not developed in the descendants of these insects. As far as I can see, the fringes, though useless, would do no harm to the species, in which they have disappeared, 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 individuals; 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 pupas 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 pupas 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 pupas, 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, will prevail, and lead to the abortion of the fringes.

I do not remember having seen, in any list of cleistogamic plants, the Podostemaceae. These curious little aquatic plants, which Lindley placed near the Piperaceae, Kunth between the Juncagineae 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.

¹ Joseph Rémi Léopold Delboeuf (1831-1896), Belgian mathematician, philosopher and psychologist. Delbœuf 1877.

² Joseph John Murphy (1827-1894), American naturalist and evolutionist who was skeptical of the sufficiency of natural selection. Murphy 1879.

[page] 464

which probably remain submerged for ever; I have not yet ascertained whether these submerged flowers are fertile; if they are so, they can hardly fail to be cleistogamic.

FRITZ MÜLLER