are at the present time constant and inherent. He has begun with a single race, and out of that single race he has produced many, differing alike from their common progenitor, and from each other. These new races breed true, i. e., steadily transmit their peculiarities by reproduction; they are independent of local and temporary circumstances; in fact, they are neither more nor less than species, and would unhesitatingly be recognized as such by naturalists if their origin were only unknown. It is unnecessary to instance particular examples, the facts are patent to every one, whether in respect to cattle, horses, dogs, fowls, vegetables, or flowers. How then has man done this? He has done it simply by availing himself of observed natural variations, which he has trusted to inheritance to perpetuate. Directly to produce variation is entirely beyond his power, he knows nothing of its causes, and can in no way influence it. He simply selects, and so controls. The variations in character in individuals of any species do not as a general rule tend to effect any specific change, if intercrossing be freely permitted, because they are perpetually neutralizing one another. But man, perceiving some variation useful to himself, isolates and preserves it by preventing the intercrossing which is calculated to destroy it. The process is repeated generation after generation, with the like precaution, until at last the variation is fixed, it has become specific. While, therefore, these facts concerning breeds prove conclusively that variation and inheritance can produce species, they show, further, that to do this a certain selection is necessary to prevent the counteracting influence of intercrosses. In the case of domestic breeds this is done by man's arbitrary isolation. Is there anything in Nature corresponding to this, and capable of producing the like effect? Undoubtedly there is. In some cases there is the very same thing at work,-isolation; a few individuals of a species are often separated locally from all others, and exposed, therefore, to but little intercrossing. If variations occurred here, there would manifestly be far greater chance, so far, of their being perpetuated and becoming specific, than in a locality where a large number were to be found together. More important, however, than this, as more generally applicable, and really more potent, is the principle which Mr. Darwin has denominated Natural Selection, and which forms the key to his whole hypothesis. All living beings reproduce themselves in a geometrical ratio of increase, which must inevitably lead to an overcrowding, a jostling, a struggle, both for position and subsistence. The fact that it is so is indisputable. What follows, then? Clearly there must be a selection perpetually

going on. Not every seed that ripens can possibly germinate, not all that germinate can grow up, not every one that grows up can come to maturity and reproduce itself; and so in like manner with animals. There is a perpetual struggle for existence going on, both among rival races and rival individuals; this struggle must lead to selection. But selection on what principle? A mere indiscriminate selection would have as little tendency to bring about specific differences in nature as an indiscriminate isolation of individuals would have to produce an improvement in breeds. The selection to be effective must be one which lays hold of particular variations, and tends to perpetuate them, to the exclusion of others. this the case here? Again we may say, from the very nature of things it must be so. The selection being mainly of the nature of a competition, it follows that just those races, those individuals, will be successful which are most perfectly adapted to the conditions under which the struggle is carried on. But the variations occurring in individuals cannot but be in many cases of considerable moment to such adaptation, either beneficially or otherwise. If the former, those individuals will be precisely such as natural selection will inevitably tend to preserve; if the latter, they will be such as natural selection will inevitably reject. The same will take place with the descendants of the favoured few, and so by a continual sifting out of those which lack the advantageous variation, or possess it in a smaller measure, the predominance of the altered form becomes yearly greater and greater, the counteracting influence of intercrossing as a consequence less and less, the variation is strengthened and rendered constant, and a specific difference is the result. Granted that species vary, that their variations frequently have a bearing on their adaptation to the circumstances of their life, that they have a tendency to transmit variations by inheritance, that there is a continual process of selection among individuals going on, which of necessity favours those possessed of advantageous variations to the exclusion of others, and there is no alternative left but to conclude that the Darwinian hypothesis is possible. The elements contained in it are real elements, their action and reaction exactly that which is asserted; the result is inevitable. The causes assigned by Mr. Darwin for the existence of specific differences, are not only real causes, such as may account for phenomena similar to those sought to be explained; but are, further, causes actually at work in the domain where these phenomena occur. Not only, therefore, is the hypothesis possible, but it is also established as to some extent true. Few, if any, probably will deny that there are some races of living beings whose

specific differences have been occasioned by such causes as those alleged by Mr. Darwin. This, however, is very far from satisfying the hypothesis, which is, not that some races have thus originated, but that all have. Mr. Darwin believes "that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number."* Analogy, indeed, would lead him "one step farther," namely, to the belief that "all animals and plants have descended from some one prototype." But this he regards as "immaterial whether or not it be accepted,"+ inasmuch as no distinct evidence can here be alleged. Confining ourselves, then, to the hypothesis proper, the descent of all living beings from some eight or ten progenitors, we proceed to our second inquiry.

2. Its adequacy. Are the causes alleged sufficient to account for all the specific differences known to exist? Here it is not pretended that more than an approximate answer can be given. It is not always possible to give even a probable account of how certain differences might thus have been occasioned. All that Mr. Darwin asserts is that his hypothesis can be shown to be adequate in so many, even of the apparently most difficult cases, that there is no valid reason on this ground for rejecting it, but rather much reason for regarding it as probably true. To estimate the validity of this position, it will be necessary to examine somewhat more in detail the extent and power of the two great elements in the hypothesis above defined-inherited variation and natural selection. The necessarily limited space of the present paper will render it, however, imperative in doing this to confine ourselves to illustrations of the kind of differences capable of being thus produced, instead of fully discussing any one or more crucial cases.

That differences in size, in colour, in detailed form, and the relative development of different parts, occur in species, and are liable to be inherited, no one probably will deny. I restrict myself therefore to variations going beyond these.

And firstly, be it noticed such variations include many striking structural changes. Thus we have such cases on record as of a woman being born with two or three toes of the right foot completely joined together with skin, partially webbed in fact; her children being free from the peculiarity, its reappearance however in some of her grandchildren, now in the foot, and now in the hand, but always on the right side; its perpetuation to her great-grandchildren in like

fashion; and so on for yet another generation. Or, again, of the absence of nails, accompanied with perfect baldness, carried down through four generations; or of hare-lip, carried down through five generations. Or, again, of deaf-dumbness, transmitted through four generations; of albinoism and other alterations in the eye, similarly hereditary. Especially do such instances prove the wonderful power of the principle of inheritance. At every successive reproduction, the influence of the original variant diminished by one half, so that by the fourth generation it amounted only to one thirty-second, by the fifth generation only to one sixty-fourth part of the total influence. Yet so strong is the tendency to reproduce variations, even when, as in these examples, of a highly disadvantageous or even abortive character, that, notwithstanding, the peculiarity still made its appearance. In a similar way the hereditary character of structural diseases, as consumption, mania, &c., is acknowledged by all. These, then, are cases where we may say everything was against the inheritance of the variation, and yet it was inherited. Had the variations been beneficial, and so themselves have tended to preservation—had, for example, the palmation of the toes occurred in a bird living partly in the water, or the baldness in another to whom head-feathers were inconvenient (and the like phenomenon has been observed to be hereditary in doves); or, again, had similar changes taken place, only in an opposite direction, -say the strengthening of the lungs instead of their weakening, or the addition of pigment to eyes formerly devoid of it, instead of its withdrawal from eyes formerly possessed of it; had especially, owing to the favourable influence of such variations, and the consequent multiplication of their possession, some of the successive generations been born of parents both of whom varied in the same manner;—had this been so, we cannot doubt but that races of living beings would have come into existence differing most markedly in structure from their progenitors, and forming species which the antiDarwinian naturalist would ridicule the idea of ever having sprung from the source they did.

Then, in the next place, it must be observed that such variations extend also to notable differences connected with habit and manner of growth. Thus no one will dispute the marked physiological distinction between a tree that sheds its leaves in the autumn and regains them in the spring, and another that retains its leaves all the year round. The internal system of such trees is manifestly widely different. Yet we have an example of a tree, the plane-tree, occasionally varying by becoming evergreen. One such in the island of Crete was famous

in Pliny's days and for long afterwards; then it disappeared; within the last few years it has reappeared, fresh shoots out of an old trunk cut down (which does not seem when standing to have been evergreen), again showing the old characteristic. This variety, we are told, could not be propagated elsewhere, the seedlings withering everywhere but in their native spot, no doubt from lack of some peculiarity in the soil or situation. We can, however, readily believe that, had the appropriate soil and situation been plentiful, this variety might have turned out its progenitor, have become confirmed in its difference, and been ranked as a remarkably distinct species.

But further, variation extends also to instincts and habits of life; acquired instincts are hereditary quite as much as natural ones. The case of the pointer is an excellent instance, the instinct of pointing being one known sometimes to occur as a variation, and being also one certainly transmissible by inheritance. It is highly probable that it was indeed originally nothing but an individual variation, become now by selection and inheritance the permanent characteristic of a race. Variations of habits in domestic animals, such as different degrees of docility, preference for particular kinds of food, fondness for various pursuits, &c., are too familiar to every one to need especially insisting on. Nor is there any doubt that such variations are to some extent hereditary. To take but a single instance, this time from creatures in a state of nature:-the dread of man, undoubtedly hereditary in many wild species, is shown, by the experience obtained in newly-peopled islands, to be an acquired, not an original instinct.

In the same way as instincts and habits may thus be occasioned by change of circumstances, as well as by natural variability, so may differences in structure and development be brought about by altered conditions of life. Every one knows how largely use and disuse tend to modify powers; few, however, probably realize the extent to which this involves also modification of structure. The superior use of the right hand and arm in man renders it the strongest and most adaptive; it also lengthens and enlarges the bones composing them. Continual practice in running will conduce to greater fleetness, which again depends in part upon the relative size of certain bones. Not only are persons born with short sight and long sight, but these can also be acquired by use. The sailor, habitually accustomed to descry distant objects, lengthens his sight, can see farther than others. The student, used to poring over his book, shortens his sight, can see nearer than others, but at the same time loses his power of seeing far off. Now what does this involve? There is in the eye a wonderful power