organization of those beings which were most affected to become, as under domestication, plastic."

Let us turn over the pages of the "Origin of Species," and see how the discussion of the problem is there elaborated. In the introduction it is pointed out that an unprejudiced naturalist, reflecting on various kinds of facts concerning organisms-such as their mutual affinities, by which is meant their morphological. resemblances and differences; their embryological relations, the appearance, in the embryo, of structures which must once have been of use in a kind of life. very different to that of the adult; their geographical distribution, as in the instance of the Galapagos Islands; and their geological succession, such as fossil sloths in South America, very different from the existing forms-might come to the conclusion that each species had not been independently created, but had descended like varieties from other species. Nevertheless, such a conclusion would be unsatisfactory, unless it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and co-adaptation which justly excites our admiration.

Naturalists continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. In one limited sense, this may be true; but it is preposterous to attribute to mere external conditions the structure, for instance, of the woodpecker, with its feet, tail, beak and tongue so admirably adapted to catch insects under the bark of trees. It is equally preposterous to account for the structure of the mistletoe, with its relation to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of

the plant itself. It is therefore of the highest importance to gain a clear insight into the means of modification and co-adaptation. To do this the author studies domesticated animals and cultivated plants.

First of all are considered the causes of variability, which, as we shall have occasion to suggest later, are as much in want of investigation now as when the

Origin of Species" was published. Cultivated organisms are more variable than those in nature: there is more difference between two individuals of a cultivated rose, than between two of the wild rose, and more difference between any two English racehorses than between any two zebras. The cause of this we are driven to conclude, is that the domesticated organisms have been raised under conditions of life not so uniform, and somewhat different from those to which the parent species had been exposed under nature. Some change is effected by the direct action of new conditions: that is, each generation, as a whole, not only some individuals, is changed from the parents in a certain direction. This is definite variability. The several breeds of the pigeon differ more than natural species from one another; yet it is almost certain that they are all descended from the rockpigeon, Columba livia. But in the formation of all domesticated breeds the principal cause has been human selection; each breed possesses characters which in some special way serve man's wants or pleasures: and the breed has been produced by man, selecting in each generation among the varying individuals those in which such characters were present in the greatest degree.

The next question is, do variations occur in a state of nature; and the answer is that no two individuals of

a given species are exactly alike. Any naturalist who examines a number of specimens of one species finds in some the "specific characters" all well marked, in others all of them less pronounced, while in others some are distinct, others less prominent. In many species there exist varieties which only differ from species in diverging in a less degree from the species to which they belong, and by being united to them by intermediate links in which the characters of the variety and the species are blended.

But the mere existence of individual variability, and of some few well-marked varieties is not sufficient to account for the exquisite adaptions of one part of the organization to another part, and to the conditions of life. Nor does it explain how varieties become converted into distinct species, and how the groups of species known as genera arise. These results follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings, and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. This is the principle of Natural Selection, called by Herbert Spencer, "the survival of the fittest." Darwin points out that the term, "struggle for existence," is used in a large and metaphorical sense. The struggle inevitably follows from the high rate at which all organic beings tend to increase, while the actual numbers of a given species never increase so fast, often decrease, and sometimes remain stationary. There is no exception to the rule that every organic being naturally increases at so high a rate, that,

if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate in less than a thousand years there would literally not be standing room for his progeny. There is direct evidence of the high rate of increase in numerous instances. The cattle and horses introduced into South America, and latterly into Australia, have multiplied to an enormous extent, and this not because the rate of breeding has increased, but because the conditions. were favourable, principally because suitable food was abundant. The exact nature of the checks to increase, of the conditions which limit the population of a given species, is obscure. The amount of food gives the extreme limit, but very frequently it is more the danger from enemies in the shape of other organisms which restricts the numbers. The complex relations in which animals and plants stand to one another when the subject is examined are seen to be marvellous. The celebrated instance given by Darwin may here be quoted :—Red clover, Trifolium protense, can only be fertilized by humble bees. The number of humble bees depends in great measure on the number of fieldmice, which destroy their combs and nests, and the number of field-mice is largely dependent on the number of cats.

Carl Vogt has pursued this chain of argument a little further. "As the red clover is one of the most important and best foods for the bullock, its quantity and quality influences the quantity and quality of beef, which is well known to be indispensable for the proper nourishment of the British nation. As further the most highly developed functions of this nation, its industry, marine, free institutions depend on the strong

development of the brains of Britons, which again depends on the abundant nourishment of these brains with beef, we find that the number of cats in England has ultimately a profound influence on the whole perfection of culture, which at the present time places Britain in many respects at the head of all

nations.”

The most difficult thing to understand, according to Darwin, is how divergence of character is produced, or when produced tends to increase. The explanation is that the greatest amount of life can be supported by greatest diversification of structure. A carnivorous quadruped can only succeed in increasing by its varying descendants seizing on places at present occupied by other animals, some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous.

By natural selection, each creature tends to become more and more improved in relation to its conditions, and then Darwin goes on to say this improvement inevitably leads to the gradual advancement of the organization of the greater number of living beings throughout the world. If we take as the standard of high organization, the amount of differentiation and specialization of the several organs in each being when adult natural selection clearly leads towards this standard; for all physiologists admit that the specialization of organs is an advantage to each being. But it is quite possible for natural selection gradually to fit a being to a situation in which several organs would be superfluous or useless.

The next chapter discusses the laws of variation, which are generalizations based on a survey of facts, though the actual causes of variation are not very