After eighteen memorable days spent with unceasing delight at Bahia, the 'Beagle' sailed again for Rio, where Darwin stopped for three months, to improve his acquaintance with the extraordinary wealth of the South American fauna and flora. Collecting insects was here his chief occupation, and it is interesting to note even at this early period how his attention was attracted by some of those strange alluring devices on the part of the males for charming their partners which afterwards formed the principal basis for his admirable theory of sexual selection, so fully developed in the 'Descent of Man.' 'Several times,' he says, 'when a pair [of butterflies], probably male and female, were chasing each other in an irregular course, they passed within a few yards of me; and I distinctly heard a clicking noise, similar to that produced by a toothed wheel passing under a spring catch.' In like manner he observed here the instincts of tropical ants, the habits of phosphorescent insects, and the horrid practice of that wasp-like creature, the sphex, which stuffs the clay cells of its larvæ full of half-dead spiders and writhing caterpillars, so stung with devilish avoidance of vital parts as to be left quite paralysed yet still alive, as future food for the developing grubs. Cases like these helped naturally to shake the young biologist's primitive faith in the cheap and crude current theories of universal beneficence, and to introduce that wholesome sceptical reaction against received dogma which is the necessary groundwork and due preparation for all great progressive philosophical thinking.
In July they set sail again for Monte Video, where the important question of climate and vegetation began to interest young Darwin's mind. Uruguay is almost entirely treeless; and this curious phenomenon, in a comparatively moist sub-tropical plain-land, struck him as a remarkable anomaly, and set him speculating on its probable cause. Australia, he remembered, was far more arid, and yet its interior was everywhere covered by whole forests of quaint indigenous gum-trees. Could it be that there were no trees adapted to the climate? As yet, the true causes of geographical distribution had not clearly dawned upon Darwin's mind; but that a young man of twenty-three should seriously busy himself about such problems of ultimate causation at all is in itself a sufficiently pointed and remarkable phenomenon. It was here, too, that he first saw that curious animal, the Tucutuco, a true rodent with the habits of a mole, which is almost always found in a blind condition. With reference to this singular creature, there occurs in his journal one of those interesting anticipatory passages which show the rough workings of the distinctive evolutionary Darwinian concept in its earlier stages. 'Considering the strictly subterranean habits of the Tucutuco,' he writes, 'the blindness, though so common, cannot be a very serious evil; yet it appears strange that any animal should possess an organ frequently subject to be injured. Lamarck would have been delighted with this fact, had he known it, when speculating (probably with more truth than usual with him) on the gradually acquired blindness of the Aspalax, a gnawer living under the ground, and of the Proteus, a reptile living in dark caverns filled with water; in both of which animals the eye is in an almost rudimentary state, and is covered by a tendinous membrane and skin. In the common mole the eye is extraordinarily small but perfect, though many anatomists doubt whether it is connected with the true optic nerve; its vision must certainly be imperfect, though probably useful to the animal when it leaves its burrow. In the Tucutuco, which I believe never comes to the surface of the ground, the eye is rather larger, but often rendered blind and useless, though without apparently causing any inconvenience to the animal: no doubt Lamarck would have said that the Tucutuco is now passing into the state of the Aspalax and Proteus.' The passage is instructive both as showing that Darwin was already familiar with Lamarck's writings, and as pointing out the natural course of his own future development.