The same amounts of light, heat, moisture, etc., produce very different effects on different species, some being much less modifiable than others. Such, for example, is the case with the ass, the guinea-fowl, and the goose, as compared with the dog, the horse, the domestic fowl, and the pigeon. As has already been remarked, * processes of repair will take place the more readily in young subjects. Similarly it is probable that the action of the environment generally acts more promptly and intensely on the embryo than on the young after birth. The modifications which may result from the action of animal agencies on the embryo have been well shown by M. C. Dareste through his experiments on eggs.t There is a well-known case of a thorough-bred brood-mare, belonging to Lord Zetland, which had a foal by a quagga, and which, though subsequently united only with thorough-bred horses, continued to throw foals with quagga marks. Analogous effects are often produced when a thorough-bred bitch has been once lined by a mongrel. These cases show that certain influences brought to bear upon a female parent, change the environment of subsequent progeny, and so induce modification. We have already cited ‡ various instances of the inheritance of modifications accidentally or artificially induced. Such cases are, however, very rare.

There is evidence that certain variations are more apt to be inherited than others. Amongst those very apt to be inherited are skin affections, affections of the nervous system and of the generative organs, eg. hypospadias and absence of the uterus. The last case is one, as before pointed out, which is especially interesting. It is often affirmed that modifications are never inherited unless they arise spontaneously, and are not produced by such influences. as use, disuse, etc., there being little good evidence of the inheritance of the latter kind of modifications. But such modifications, depending as they often do on changes in deep

* See above, p. 170.

† See "Archiv. de Zool. Exper.," vol. ii. p. 414; vol. v. p. 179; vol. vi. p. 31 ; also “ Ann. des Sc. Nat.," 4th series, "Zoologie," vol. iii. p. 119; vol. xv. p. 1; vol. xvii. p. 243; and his work entitled, Recherches sur la Production Artificielle des Monstrosités."

See above, p. 341.

66

§ See above, p. 174.

lying structures, are not easily to be noticed, and there has hitherto been no motive for seeking out and attending to them. Some evidence, as we have just seen, moreover, there absolutely is. But such objectors may well be asked, What are "spontaneous variations"? No effect can be produced without some cause, and the objectors referred to would never affirm that the cause of variation was an absolutely internal one. It must, then, in some way be due to the environment and to reaction between the environment and the organism (nascent or adult). We must attribute the special formation of the water-hen of Tristan d'Acunha, to inherited modifications of the kind. Thus, to a greater or less extent, not only single individuals, but whole races are modifiable by the direct action of the environment, such modification being in some cases most certainly inherited. The facts mentioned in this chapter, show that the geographical and geological relations of organisms harmonize with the view that a blood-relationship has existed between successive species of different strata, and co-existing forms of different regions. It also appears that in the struggle for existence, small differences. of colour, form, or strength, may not only prolong the lives of individual animals, but also enable them to mate and breed. At the same time it is evident that surrounding conditions operate directly, and may even occasion a nonprotective mimicry. Many species of animals are defined t by the possession of apparently indifferent or even slightly disadvantageous characters. The bearings of these facts will occupy us in the last chapter.

With the statements of fact made in this and the preceding section, we terminate our rapid survey of the world and its inhabitants. We must next endeavour to ascertain what are the most general and ultimate scientific truths which the facts of nature, ascertained by the physical sciences, may be able to disclose to our intellect enlightened by those fundamental principles which we have seen to be absolutely and necessarily true. So we may hope to ascertain what things are evidently and what most probably true, and to learn "what is truth" as regards the nature and principles of action of the material universe and our own being.

*

See above, p. 373.

E.g. the Potto (see p. 317).

CHAPTER XXV.

NATURE.

A mechanical conception of nature, though useful as a working hypothesis, is in reality an irrational conception. Reason indicates that the world is made up of bodies, each of which is a compositum consisting of both matter and an active, immaterial principle; the two forming a substantial unity.

What science is-What we are-The soul-The correlation of forcesThe mechanical philosophy-The undulatory theory of light-The nature of gases—The atomic theory—The nebular theory-Space -Time-Motion-Energy-Matter-Motion and thought-The inorganic world—The organic world-Animal automatism— What an organism is-Psychology and physiology-Organic symmetry-Vitalism-Five orders of immaterial principlesGeneration-Intermediate temporary forms-Cosmical hypotheses -Existences, real and ideal—Reason in nature.

OUR pursuit of truth has now carried us through a brief survey of the world about us, and we have noticed many of the leading phenomena which form the subject-matter of various sciences. We have next to advance from the sciences, to the science of sciences, or science par excellence. Our endeavour must now be to obtain what knowledge we may of the highest truth which appears to us attainable by man's natural faculties. And here it may be well to repeat what was said at the outset of this work,* namely, that the inquirer after truth must trust only to the dictates of his own reason, and be careful to accept nothing as certain, except what his intellect, after patient and persevering thought, shows him to be evidently true. That in such an *See above, p. 4.