sages only partially obscure, possess less atrophied optic apparatus. A singular gradation occurs among the burrowing mammals, and Darwin 3 cites an example admirably illustrating the loss of sight in consequence of the mode of life.. "In South America a burrowing rodent, the Tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind; one which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been the inflammation of the nictilating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and, if so, natural selection would constantly aid the effects of disuse."

In the classes of flying animals, a large number have left off flying; and we find their flying apparatus in an aborted or incomplete condition, which perverse judgment and reasoning alone can regard as a state of progressive development from yet simpler rudiments. If throughout the great family of the Coleoptera, genera and species are to be found with imperfect flying apparatus, consolidated wing covers, &c., if the whole family of Staphylinæ does not possess the power of flight, no one dreams of considering them as arrested forms; but it is conceivable that the mode of life in which they differ from the other members of their order and class, gradually superinduced in their flying ancestry the habit of not flying, and at the same time the atrophy

of the organs of flight. With this was combined, as these beetles show, no degradation of organization, but, on the contrary, a higher and extremely advantageous development of other organs, the manducatory and locomotive apparatus. A general reduction of the power of flight has been shown in the beetle fauna of many islands. Thus in Madeira, of 550 species, over 200 fly imperfectly or not at all, and for this there is no explanation but natural selection. Here the less. good and enterprising flyers had the advantage, while the others were blown into the sea and eliminated. The non-application of a previously attained special perfection is advantageous in the "struggle for existence."

In several families of lizards, some genera are serpentine, as they are termed, which, with elongated bodies, possess either fore-legs only (Chirotes), or merely rudimentary hind-legs (Pseudopus), or no vestiges of legs (Anguis). They bear the same relation to the great class of normally four-legged lizards as the non-flying insects to their own class. They have not been arrested in their development, nor are they animals in process of evolving four legs; but, as Fürbringer has demonstrated from the history of development and comparative anatomy, their limbs, and—if these are entirely absent-the remains of the pectoral and pelvic arches and the sternum bear indubitable marks of the abortion of a once complete apparatus. Further comparison shows that this atrophy reaches its climax in the snakes, but that it is compensated for by the ribs and intercostal muscles having undertaken the work of the limbs. Here,

again, disuse and adaptation coincide as well as differentiation.

In the class of birds is repeated the spectacle we have just witnessed in beetles and reptiles. In some few families and smaller groups, individual species are deprived of the power of flight, and one whole large systematic group is characterized by the incapacity of flying. In our opinion, there was a direct connection between the inducements to disuse and its consequences in the case of the dodo, which, with its few congeners, so promptly fell a sacrifice to its helplessness on the discovery of the lonely islands which they had probably inhabited for thousands of years without disturbance. In no other way has the northern penguin (Alca impennis) at some time obtained the curtailment of its wings; and the scanty but wide-spread remains of the order of flightless birds indicate a period at which, in a more peaceful environment, their far more numerous wingless ancestry made less use of their pinions, and natural selection endowed them with greater strength and nimbleness of leg. The effects of disuse of the organs of locomotion are likewise directly exhibited by artificial selection.

Use and disuse, combined with selection, elucidate the separation of the sexes, and the existence, otherwise totally incomprehensible, of rudimentary sexual organs. In the Vertebrata especially, each sex possesses such distinct traces of the reproductive apparatus characteristic of the other, that even antiquity assumed hermaphroditism as a natural primæval condition of mankind. The technical proofs of the homologies concerning these partly manifest, partly internal and hidden relations, are

given in the manuals of comparative anatomy. We shall merely indicate the manner in which the theory of selection is here borne out. It is self-evident that in hermaphrodite animals, fluctuations in the sexual sphere must take place, in which one half or the other will predominate. Should these fluctuations be sufficiently strong for natural selection to take possession of them, the productive power of the less active portion will gradually decrease, and finally, with the extinction. of the physiological character and the function, nothing will be transmitted but the morphological remains, as a mockery to the theory of special design or teleology. Here and there only occurs a reversion more or less striking, connected, however, almost exclusively with the adjunctive organs, and the secondary sexual characters, by which we mean, not those acquired by either sex, but originally common to both. The tenacity with which these rudiments of sexual organs are inherited is very remarkable. In the class of mammals actual hermaphroditism is unheard of, although through the whole period of their development they drag along with them these residues, borne by their unknown ancestry no one can say how long.

Unless we suppose that parasitic animals were created simultaneously with their hosts from the dust of the earth,-man and his tapeworm, and other disagreeable guests, and thus put an end to the discussion, this entire province has to be explained by descent, with the special co-operation of disuse. The proposition to be demonstrated in the next chapter, that the evolutionary history of the individual represents the history of the species, will show the influence of the disuse of

particular organs on the configuration of the various parasites. The parasitic Crustacea are perhaps the most instructive, as they present the most complete systematic series, exhibiting the gradual atrophy of the organs which accompanies the ever-increasing connection of the parasite with his host. In several orders of intestinal worms, the alimentary canal has become entirely unnecessary; but they exhibit neither intermediate forms nor phases of development. It is different, however, with the parasitic Crustacean, for here the young, locomotive, and well-integrated being has its prototype in definitive generic forms permanently locomotive, from which, after adhesion, it deteriorates into a mere motionless sac. All these animals, including the intestinal worms, have acquired their position and status (and this is the true significance of parasitic life) by the apparent degradation of their organization. They are, almost without exception, distinguished by their reproductive power; and on this, owing to the easy supply of nutriment, without any exertion of the other parts of the organic system, the whole bodily activity could be concentrated.

We have hitherto demonstrated that organisms are urged to continual differentiation by the unremitting struggle for existence. For the cultivation of morphological species, natural selection, moreover, seizes on the modifications arising from the mere variability of the organism, and implying no physiological advance. But sooner or later these are also inevitably drawn into the vortex of competition. After what has been already said, this fact is so self-evident as to need no further proof. Even did we not see the infinite variety of