is to say, it gives out its light and heat into space and becomes dark, until at length it comes to form one of the constituents of a still more stupendous collision, and has its temperature raised once again by the conversion of visible energy into heat.

163. Our readers will remark how, by a process of this kind, the primordial potential energy of the visible universe is gradually converted into light and heat, and how this light and heat are ultimately dissipated into space. They will also remark that, as the process goes on, the masses of the universe become larger and larger. In fine, the dissipation of the energy of the visible universe proceeds, pari passu, with the aggregation of mass.

The very fact, therefore, that the large masses of the visible universe are of finite size, is sufficient to assure us that the process cannot have been going on for ever; or, in other words, that the visible universe must have had its origin in time, and we may conclude with equal certainty that the process will ultimately come to an end. All this is what would take place, provided we allow the indestructibility of ordinary matter; but we may perhaps suppose (Art. 153) that the very material of the visible universe will ultimately vanish into the invisible.

164. There is one peculiarity of the process of development now described, which we beg our readers to note. We have supposed the visible universe, after its production, to have been left to its own laws; that is to say, to certain inorganic agencies, which we call forces, in virtue of which its development took place. At the very first there may only have been one kind of primordial atom, or to use another expression, perfect simplicity of material. As, however, the various atoms approached each other, in virtue of the forces with

which they were endowed, other and more complicated structures took the place of the perfectly simple primordial stuff. Various molecules were produced at various temperatures, and these ultimately came together to produce globes or worlds, some of them comparatively small, others very large. Thus the progress is from the regular to the irregular. And we find a similar progress when we consider the inorganic development of our own world. The action of water rounds pebbles, but it rounds them irregularly; it produces soil, but the soil is irregular in the size of its grains, and variable in constitution. Wherever what may be termed the brute forces of nature are left to themselves, this is always the result; not so, however, when organisms are concerned in the development. Two living things of the same family are more like each other than two grains of sand or two particles of soil. The eggs of birds of the same family, the similar feathers of similar birds, the ants from the same ant-hill, have all a very strong likeness to each other.

This likeness is still more marked if we regard certain products of human industry. Let us take, for instance, coins from the same die, or bullets from the same mould, or impressions from the same engraved plate, and we at once perceive the striking difference between products developed through inorganic means and those developed through an intelligent agent designing uniformity.

165. Let us now proceed to consider life development. Let us imagine that the primeval atoms have long since come together, various chemical substances being the result. And let us further imagine that these various substances have long since gathered themselves into worlds, of various sizes at first; but these worlds have gradually cooled down,

until one of them, the Earth, let us say, has at length reached conditions under which life (such as we know it) becomes possible. Accordingly life makes its appearance; not the life that now is, but something much ruder and simpler. But in process of time we find quite a different order of organised beings; a higher and more complete type has appeared, and the type continues to rise until it culminates in the production of man, a being endowed with intelligence, and capable of reasoning upon the phenomena around him. Now, if man reviews these organised forms which exist on the earth side by side with himself, he perceives at once that a number of individuals possess certain characteristics in common, and he gives expression to this experience by saying that these individuals are all of one species. "When we call a group of animals or of plants a species," says Professor Huxley, "we may imply thereby, either that all these animals or plants have some common peculiarity of form or structure; or we may mean that they possess some common functional character. That part of biological science which deals with form and structure is called Morphology; that which concerns itself with function, Physiology. So that we may conveniently speak of these two senses, or aspects, of 'species'—the one as morphological, the other as physiological. . . . Thus horses form a species, because the group of animals to which that name is applied is distinguished from all others in the world by the following constantly associated characters :-They have, 1. a vertebral column: 2. mammae ; 3. a placental embryo; 4. four legs; 5. a single well-developed toe in each foot, provided with a hoof; 6. a bushy tail; and 7. callosities on the inner sides of both the fore and the hind legs. The 1 Lay Sermons, Essays, and Reviews.

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asses, again, form a distinct species, because, with the same characters, as far as the fifth in the above list, all asses have tufted tails, and have callosities only on the inner side of the fore legs."

But very often the morphological peculiarities of a species are more easily recognised than expressed. No one, for instance, would fail to rank the horse as one species and the ass as another, even while ignorant of some of those specific peculiarities which the naturalist selects as conveying the best scientific account of their difference.

166. Let us now regard the question of species from its physiological point of view. Suppose that two individuals, A and B, of different sexes, breed freely together, producing offspring, and that two individuals, C and D, do the like.

Now, if the offspring of A and B is capable of breeding freely with that of C and D, producing offspring, generation after generation, then A, B, C, and D may be said to belong to the same physiological species.

To take an illustration borrowed from Professor Huxley: let us imagine that A is an Arab, and B a dray-horse; also that C is a dray-horse, and D an Arab. Now the progeny of these two pairs will all be mongrels, holding a position intermediate between that of the Arab and the dray-horse; but they will be perfectly fertile amongst themselves when matched together. We therefore conclude that the drayhorse and the Arab are not distinct physiological species, but only varieties of the same species. Again, let A be a horse and B an ass, also let C be an ass and D a horse. The pairs will still have offspring, and these will be mules, having a character intermediate between that of the horse and that of the ass; but, on the other hand, these mules will not be able to breed together amongst themselves so as to

produce offspring. We are therefore justified in asserting that a horse and an ass are of different physiological species..

If we should ever attempt to pair together animals much more unlike each other than the horse and the ass, we should simply fail. They will not come together, and we cannot tell whether, if they did, they would be capable of producing progeny. We may therefore conclude that, as matter of fact, there are certain well-marked physiological species that will not breed with each other at all, while there are other species also physiologically distinct, but not so markedly separated from each other, that may be brought to breed together, their offspring being infertile.

167. The most apparent conclusion to be deduced from these facts would be that of the invariability of species, and of the impossibility of its transmutation—the infertility of hybrids being the law that prevents any such transmutation taking place. And as the physiological species cannot be made different, the apparent conclusion is that in times past they have been always the same as they are now. If this be allowed, it follows that inasmuch as they took their origin in time, they must have originally been produced very much as they are at the present moment,-a separate act of production being required for each species, or rather two separate acts for each species. This position has always been regarded as a stronghold by a certain class of theological thinkers, and they have resented the attempts of men of science to obtain any other explanation of the origin of species.

Men of science have, on the other hand, asserted their right to discuss this question with the same freedom as any other. Our point of view is somewhat different from that of either of these two parties. We think it is not so much