steady and continuous, though slow, progress upwards. It is then asked, how is it that even at the present day we find numerous representatives of the simplest groups of animals living? And how is it that we find many cases of degeneration-i.e., of animals which, in the early stage of their existence -representing ancestral phases are more highly organised than in the adult condition ?

Now, an animal may be placed under conditions in which organs useful to its ancestors, and inherited from them, may be no longer of service. Such organs tend to become degenerate, persisting for a time as vestigial structures, and ultimately perhaps disappearing altogether. Of such cases of degeneration we meet with numerous examples, of which the following are the most important:

(a) An animal fixed in the adult state, but free when young: such as sponges, hydroids, corals, polyzoa, oysters, and barnacles. This involves loss, or modification, of the locomotive organs, and often of the sense-organs as well.

(b) Parasites which live on or in other animals, and of which Sacculina is a good example. In these animals the whole body often becomes degenerate, the conditions of life rendering locomotor, digestive, sensory, and other organs entirely useless. In such case cases, those forms which avoid the waste of energy resulting from the formation and maintenance of these organs will be most in harmony with their surroundings. Parasitic worms, molluscs, &c., show similar wholesale degeneration, and live immersed in the body fluids of their victims.

The explanation of the extreme degeneration of parasites is that special food is required to meet the drain at the time of ripening of the eggs. For instance, in Copepoda, the female is alone parasitic, and that only at the time of laying eggs. The new phase intercalated in the life-history involves the necessity of laying more eggs, and there is greater difficulty in completing the ancestral history in individual development. This reacts on the parasitic stage, rendering it more important; more food is required, and hence further modification

ensues.

(c) Special organs show signs of degeneration even in the highest animals, and give evidence of a former more perfect condition in their ancestral forms. This is seen in the eyes of the mole, and in many cave animals; in the splint bones of the horse, and in all the examples of rudiments or vestiges mentioned in a former lecture.*

In a sense, all the higher animals are degenerate; that is, they can be shown to possess certain organs in a less developed condition than their ancestors, or even in a rudimentary state. Thus, a crab, as compared with a lobster, is degenerate in regard to its tail; a horse, as compared with Hipparion, in regard to its outer toes. It is a mistake, however, to speak of a crab as a degenerate animal in comparison with a lobster, for an animal should only be spoken of as degenerate when the retrograde development has affected, not one or two organs only, but the totality of its organisation.

* See page 95.

L

No animal is at the top of the tree in all respects; man himself being primitive in retaining the full number of toes, and degenerate as regards his ear muscles. Care must also be taken not to speak of an animal as degenerate merely because it possesses organs less fully developed than allied animals. An organ is not degenerate unless its present possessor has it in a less perfect condition than its ancestors had. A man is not degenerate in the matter of the length of his neck as compared with a giraffe, nor as compared with an elephant in respect of the size of his front teeth, for neither elephant nor giraffe enters into the pedigree of man. A man is, however, degenerate, whoever his ancestors may have been, in regard to his ear muscles, for he possesses them in a rudimentary and functionless condition, which can only be explained by descent from some better equipped progenitor.

The theory of Natural Selection does not say that the ideally best survive, but those most in harmony with their surroundings for the time being. If these are of such a kind as to render certain organs useless, such as the eyes of cave dwellers, their possession is no longer an advantage, and the energy previously devoted to their production can be better utilised in other directions. Hence, though it is quite true that on the whole there has been a progress towards greater specialisation, and that differences between extreme groups are greater now than ever, yet there are many individual exceptions, and natural selection actually requires that there should be such exceptions.

THE ALLEGED USELESSNESS OF SMALL
VARIATIONS.

This is an objection which has often been put forward. Admitting that no two animals are absolutely identical, it is urged that the differences are in most cases too small and too trivial to have the effect assigned to them; namely, to determine between survival or destruction. This is, however, a misapprehension, for if four out of five are to die, a very small matter may determine success or failure.

In a race for which there is but one prize, a victory by a short head is, so far as securing the prize is concerned, as conclusive as a win in a canter. Again, the whole theory and practice of the breeding of animals and plants afford absolute proof of the importance of attention to minute details, so slight as to escape the notice of all but the most skilful observers.

Think how in commerce a very small and subordinate point may determine survival; such as, for instance, the use of bye-products resulting from certain chemical manufactures, which had previously been neglected and regarded as waste products. Think what small events have decided the fate of battles and of nations. "The death of a man at a critical juncture, his disgust, his retreat, his disgrace, have brought innumerable calamities on a whole nation. A common soldier, a child, a girl at the door of an inn have changed the face of fortune and almost of nature."

:

THE DIFFICULTY WITH REGARD TO THE EARLIEST COMMENCEMENT OF ORGANS.

This difficulty is a very serious one, for Natural Selection can only act on an organ after it has already attained sufficient size to be of practical importance and utility. Natural Selection accounts for any amount of modification in an organ when

once

established; modification in any direction, either of increase or decrease, but does not offer any explanation of the first appearance of such an organ. This is best understood by a few examples, showing the continuous preservation of a series of very minute variations.

1. The wing of the Bat, a flying mammal, is clearly a modified arm with great elongation of the fingers and webbing of the skin, which also extends from the side of the body and involves the hind-legs and tail (Fig. 28). It is easy to see that, when once established as a flying organ, Natural Selection would cause survival of those with the best wings, and so lead to gradual improvement and perfection of the wing. But how does the wing first commence ?

Bats are a specialised group of mammals which must have been descended from non-flying ancestors. If the first commencement of the wing was a slight accidental elongation of the fingers, and a slight increase in the webbing, this would not give the power of flight, and would be of no use as a wing until it had attained a considerable size. In other words, such an organ as a wing would in its earliest