& that their loss, or semiobliteration has resulted first from a combination of circumstances, bringing Disuse into play as an active agent,= 2^dly by a perpetuation of the misfortune by inheritance,—till what was first accidental becomes normal— Now if the darkness of habitual residential places,— (burrows deep caves & the like), produces this effect in certain cases, why not in all where the like causes prevail?— But such is not the case.²

Look for example at your little Tuco=Tuco—(Ctenomys Brasiliensis)—³ this subterranean creature is not blind, although it often loses its sight by disease—(as the English soldiers in Egypt lost theirs from the irritation of impalpable sand)—nor is our mole,—although, if I remember rightly its optic nerve is either rudimentary or wanting, a fibril of the 5^th pair appearing to supply its place— neither is the Hamster blind= But the Diplostoma which lives under ground, on roots, in the plains of the Missouri, appears to be so,—its eyes being covered by hair—⁴

In the Bathyergus, of the Cape, the eyes are uncovered, though very minute—⁵

In the Mole-rat of Asia minor—& the steppes of Tartary—(Asphalax Typhlus, Desmarest) the specks representing eyes are concealed beneath the skin—this subterreous creature is truly sightless—⁶

If we turn from these rodents to certain burrow dwelling Insectivora we shall find that eyes, though often very minute are not wanting— we may instance our shrew mouse (Sorex)—the Desman (Mygale G. Cuv:) of Muscovy, (also Pyrennees) in the Talposorex of N: America in the Cape Chrysochloris, & the Condylura of N. Amer. &^c.—⁷

Now may we not rather attribute the non=developement or even nullity of eyes in these & other instances, to non=necessity for them, their presence in perfection being of no service, than to Disuse, which latter seems to me to say that eyes were once in perfection—whereas I think they have been stinted or arrested in their development, nature never wasting her resources,—& therefore stopping where her end is accomplished.— I am not however dogmatic on this point— Much may be said on both sides.—

But let us return to your Tucotuco (so interestingly described in your Journal⁸ a work more delightful to me than any work of the kind I have ever read, & a model for travelling naturalists)— Let us, I say return to your Tucotuco— We learn (& I remember examining the specimen dissected by M^r Reid)⁹ that its thigh bone is destitute of the Ligamentum teres,— accounting for its queer movements.— Now this ligament is wanting in very few mammals,—(but I am not aware of its absence in any birds) among which are four of very different character—viz the Orang, & the Elephant—The Seal tribe & the 3=toed Sloth. (I have mislaid my notes on this subject, & therefore hesitate to name any others)—

What are we to say respecting this absence, in such diverse examples.— now Animals as ponderous & lourd in motion as the Elephant, as good climbers as the orang & as efficient burrowers as the Tucotuco, do possess this ligament (so needful in man) thoroughly developed— Surely we cannot attribute these respective deficiencies to Disuse— Is it not because there is here no need of this ligament, or perhaps that it would be worse than unserviceable that Nature has not given it?— Is there not a negation in nature, when a priori, we should not anticipate (scrutiny alone teaching us) such a structural peculiarity.— Consider the double cæcum of Birds (present & single in man the Orang & one or two more)— Does not anatomy reveal strange facts? structural facts which we cannot, as I think attribute to disuse—& totality of anatomical structure must be the basis of our speculations— Can Disuse thoroughly alter the great points of anatomy— or take away the cells of the camel’s stomach in that of the ox?— In this latter instance has natural selection obtained & perpetuated them?—that is in the Camel— most probably—

But then what shall we say to the Ligamentum teres?— Has Natural Selection adopted it in the majority of mammals, & neglected it in the few;—or has Disuse, or the non=necessity of it operated so as to prevent its development= Is Natural selection confined only to adoption? or does it involve rejection?—