over his fences, it occurred to him that, if he could produce a breed of sheep with short bandy legs, he would lose none of them, as they would be unable to jump his fences. He therefore bred entirely from the short-legged ram when it had reached maturity, and after a few years succeeded in raising a considerable flock of this variety, which was known as the Ancon sheep.

The power of artificial selection is almost unlimited, and breeders of animals speak with the utmost confidence of being able to produce any desired result in the form of the body; and, in the case of poultry and pigeons, in the length of beak, the number of feathers, and even in the markings on particular feathers. Lord Spencer says: "It is therefore very desirable before any man commences to breed either cattle or sheep, that he should make up his mind as to the shape and qualities he wishes to obtain, and strictly pursue this object." And speaking of Leicester sheep Lord Somerville remarks: "It would seem as if they had just chalked on the wall a form perfect in itself, and then had given it existence." So also with plants; enormous changes are effected by cultivation-i.e., by selection, in fruits and flowers.

NATURAL SELECTION.

The theory of Natural Selection teaches that there are in Nature causes which act in much the same way as man acts when selecting artificially the best animals for breeding purposes; causes which must lead to structural modifications; and that this is the clue to the unlikeness between the fauna of successive geologic ages.

I propose first to give an outline of the argument and to consider the arrangement given in Mr. Wallace's chart of the Theory of Natural Selection.

A. All animals produce far more young than can survive. Consider for instance the enormous number of eggs of fish or oysters.

B. The total numbers are on the average stationary. As a necessary consequence of this there is a struggle for existence, because there is neither enough space nor food for all.

D. Variation with heredity. No two animals are exactly alike, and their distinctive characters are transmitted from generation to generation. The consequence of this is the survival of the fittest-that is, that in the long run those best adapted to their circumstances and environment will have the best chance of surviving, and of leaving descendants who will hand down their peculiarities. Just as man selects artificially the forms best suited for his purpose, and by breeding from them produces great changes in structure and habit, so in Nature the best and fittest of each generation have an advantage and the best chance of survival.

F. Change of environment, rendering old characters of less value and bringing new ones to the fore. From this follow-Structural modifications.

Causes are always at work which must lead to change in structure, and this to an apparently unlimited extent.

Let us now examine the argument more closely. A. RAPID INCREASE OF ORGANISMS. "There is no exception to the rule that every organic being, animal or plant, naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair." Man himself has doubled his numbers in the United States in the course of twenty-five years, and at this rate in less than 1000 years there literally would not be standingroom on the earth for his progeny. Linnæus showed that an annual plant producing two seeds only-and there is no plant so unproductive as this and these each producing two in the following year, and so on, would in twenty-one years produce over a million plants, as shown in the following table :

The rate of increase of an animal, each pair producing ten pairs annually, and each animal living ten years, is shown in the following table :

Vast numbers of eggs are laid by some animals ; the conger-eel, for instance, lays 15 millions; the herring 20,000; the oyster from half a million to 16 millions; and a very large oyster may produce even 60 millions of eggs. Supposing we start with one oyster and let it produce 16 million eggs, the average American yield, and let half, or eight millions, be female and go on increasing at the same rate; in the second generation we shall have 64 millions of millions of female oysters. In the fifth generationi.e., the great-great-grandchildren of our first oysterwe should have 33 thousand millions of millions of millions of millions of millions of female oysters. If we add the same number of males we should have in all 66 + 33 noughts. If we estimate these as oyster-shells, we should have a mass more than eight times the size of the world.

A large number of eggs or young is, however, not essential. The Fulmar petrel lays only one egg, yet it is believed by Darwin to be one of the most numerous birds in existence. The Passenger pigeon again only lays two eggs, yet it is extraordinarily abundant in parts of North America, where its enormous migrating flocks darken the air for hours. A remarkable account is quoted by Wallace of a wood in Kentucky, 40 miles in extent, where there was a perpetual tumult of crowding and fluttering pigeons, and where there were as many as a hundred nests on a single tree, the branches of which were often broken off by their weight, and the ground strewn with broken limbs of trees, eggs, and young birds, on which herds of hogs were fattening. Hawks, buzzards, and eagles were flying about in large numbers, seizing the young birds at pleasure; and numerous parties of men from all parts of the adjacent country were camping with their families for several days, felling trees to get the nests.

Another good example of rapid increase in numbers was seen in the rabbit pest of Australia in 1887. The common grey variety of wild rabbit