shape, whilst the cloacal chamber forms. The brain remains quite small and undeveloped, and the remarkable myelonic eye (the eye in the brain) disappears. The number of gill-slits increases as the animal grows in size and its outer skin becomes tough and leatherlike.

Before saying anything further on the subject of degeneration, it seems desirable once more to direct

FIG. 21.-Section through the eye ("surface eye") of a Water-beetle's larva. All the cells are seen to be in a row continuous with h, the cells of the outermost skin or ectoderm. p, pigmented cells; r, retinal cells connected at o with the optic nerve; g, transparent cells (forming a kind of "vitreous body"); 7, cuticular lens. (From Gegenbaur's Elements of Comparative Anatomy, after Grenacher.)

FIG. 22.-Section through the eye ("surface-eye") of a Marine Worm (Neophanta). , integument spreading over the front of the eye c; l, cuticular lens; h, cavity occupied by vitreous body; p, retinal cells; b, pigment; o, optic nerve; o', expansion of optic nerve.

attention to the myelonic or cerebral eye which the Ascidian tadpole possesses in common with all Vertebrates. All other animals which have eyes develop the retina or sensitive part of the eye from their outer skin (see Figs. 21 and 22, and explanation). It is

easy to understand that an organ which is to be affected by the light should form on the surface of the body where the light falls. It has long been known as a very puzzling and unaccountable peculiarity of Vertebrates, that the retina or sensitive part of the eye grows out in the embryo as a bud or vesicle of the brain, and thus forms deeply below the surface and away from the light (see Fig. 23, and explanation). The Ascidian tadpole helps us to understand this, for

FIG. 23.-A. Vertical section through the head of a very young fish, showing in the centre the cavity of the brain c. On each side is a hollow outgrowth (a) which will form the retina of the fish's eye ("cerebral eye"); b will become the optic nerve connecting the brain and the retina; d, integument.-B. Later condition of the hollow outgrowth (a) of A. Its outer wall r is pressed against its deeper wall p by an ingrowth (7) from the outer skin (ectoderm) e; r gives rise to the retinal cells, whilst only l, the cellular lens, is derived from the surface of the skin.

it is perfectly transparent and has its eye actually inside its brain. The light passes through the transparent tissues and acts on the pigmented eye, lying deep in the brain. We are thus led to the suggestion -and I believe this inference to be now for the first time put into so many words-that the original Vertebrate must have been a transparent animal, and had an eye or pair of eyes inside its brain, like that of the

Ascidian tadpole. As the tissues of this ancestral Vertebrate grew denser and more opaque, the eyebearing part of the brain was forced by natural selection to grow outwards towards the surface, in order that it might still be in a position to receive the influence of the sun's rays. Thus the very peculiar mode of development of the Vertebrate eye from two parts, a brain-vesicle (Fig. 23, A a, and B p r) and pr) a skin-vesicle (Fig. 23, B e, l), is accounted for.1

The cases of degeneration which I have up to this point brought forward, are cases which admit of very little dispute or doubt. They are attested by either the history of the individual development of the organisms in question, as in Sacculina, in the Barnacle, and in the Ascidian, or they are cases where the comparison of the degenerate animal with others like it in structure, but not degenerate, renders the hypothesis of degeneration an unassailable one. Such cases are the Acarus or mite, and the skin-worm (Demodex).

We have seen that degeneration, or the simplification of the general structure of an animal, may be due to the ancestors of that animal having taken to one of two new habits of life, either the parasitic or the immobile. Other new habits of life appear also to be such as to lead to degeneration. Let us suppose a race of animals fitted and accustomed to catch their food, and having a variety of organs to help them in this chase-suppose such animals suddenly to acquire the

1 I do not at the present time (November 1889) attach great importance to the above suggestion. The facts admit of other possible and plausible interpretations.

power of feeding on the carbonic acid dissolved in the water around them just as green plants do. This would lead to a degeneration; they would cease to hunt their food, and would bask in the sunlight, taking food in by the whole surface, as plants do by their leaves. Certain small flat worms, by name Convoluta, of a bright green colour, appear to be in this condition. Their green colour is known to be the same substance as leaf-green; and Mr. Patrick Geddes has recently shown that by the aid of this green substance they feed on carbonic acid, making starch from it as plants do. As a consequence we find that their stomachs and intestines as well as their locomotive organs become simplified, since they are but little wanted. These vegetating animals, as Mr. Geddes calls them, are the exact complement of the carnivorous plants, and show how a degeneration of animal forms may be caused by vegetative nutrition.

Another possible cause of degeneration appears to be the indirect one of minute size. It cannot be doubted that natural selection has frequently acted on a race of animals so as to reduce the size of the individuals. The smallness of size has been favourable to their survival in the struggle for existence, and in some cases they have been reduced to even microscopic proportions. But this reduction of size has, when carried to an extreme, resulted in the loss or suppression of some of the most important organs of the body. The needs of a very minute creature are limited as compared with those of a large one, and thus we find heart and blood-vessels, gills and kidneys,

may

besides legs and muscles, lost by the diminutive degenerate descendants of a larger race. That this is a

possible course of change all will, I think, admit. It is actually exemplified in Appendicularia-the only adult representative of the Ascidian tadpole-still tadpole-like in form and structure, but curiously degenerate and simplified in its internal organs. This kind of degeneration is also exemplified in the Rotifers, or wheel animalcules, in the minute Crustacean waterfleas (Ostracoda), and in the Moss-polyps, or Polyzoa. Roughly then we may sum up the immediate antecedents of degenerative evolution as, 1, Parasitism; 2, Fixity or immobility; 3, Vegetative nutrition; 4, Excessive reduction of size. This is not a logical enumeration, for each of these causes involves, or may be inseparably connected with, one or more of the others. It will serve for the present as well as a more exhaustive analysis. (See Note C.)

And now we have to note an important fact with regard to the evidence which we can obtain of the occurrence of this process of degeneration. We have seen that the most conclusive evidence is that of the recapitulative development of the individual. The Ascidian Phallusia shows itself to be a degenerate Vertebrate by beginning life as a tadpole. But such recapitulative development is by no means the rule. Quite arbitrarily, we find, it is exhibited in one animal and not in a nearly allied kind. Thus very many animals belonging to the Ascidian group have no tadpole young-just as some tree-frogs have no tadpoles. It is quite possible, and often, more often than not,