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excentricity of the earth's orbit. The earth goes through a cycle of temperate seasons, and seasons extreme in their heat and cold; and this cycle (of 21,000 years) itself undergoes exaggeration and mitigation during epochs that are far longer. If, then, the yearly change of seasons produces changes in organisms, and prompts or compels the migration of birds to other zones, and of fishes from one part of the sea to another, or from salt water to fresh, these alternations of temperate and intemperate climates must produce great structural changes, and give rise to extensions and restrictions of habitat.

The power of geological actions to modify everywhere the circumstances in which plants and animals are placed is conspicuous. In each locality denudation slowly uncovers different deposits, and slowly changes the exposed areas of deposits already uncovered. The inclinations of land surfaces, and their directions with respect to the sun, are at the same time altered, and the living things existing on them are thus subjected to continual alteration in their supply of heat as well as their drainage. Igneous action, too, complicates these gradual modifications; for a flat region cannot be step by step thrust up into a protuberance, without dissimilar climatic changes being produced in its several parts by their exposures to different aspects. In like manner, alterations in the earth's crust cause new combinations of conditions to living things inhabiting the ocean. Here the water is being deepened by subsidence, and there shallowed by upheaval. The mineral character of the submerged surface on which sea-weeds grow and molluscs crawl, is changed by the addition of material from an adjacent shore, or the accumulation of organic

remains―pteropods or foraminifera. Each modification in the outlines of neighbouring shores makes the tidal streams vary in their directions or velocities, or both, and so alters the circumstances in which marine organisms live. These and other geologically-caused changes in the physical characters of each environment occur in ever-new combinations, and with ever-increasing complexity.

Changes in the astronomical conditions, joined with changes in the geological conditions, bring about others of a meteorological character. While yet the highest parts of an emerging surface of the earth's crust exist as a cluster of islands, the plants and animals which in course of time migrate to them have climates that are peculiar to small tracts of land surrounded by large tracts of water; but as, by successive upheavals, greater areas are exposed, there begin to arise sensible contrasts between the states of their sea-board parts and their central parts. The sea and land breezes which daily moderate the extremes of temperature near the shores cease to affect the interiors; and the interiors, less qualified also in their heat and cold by such ocean currents as bathe the shores, acquire more decidedly the characters due to their latitudes.

Besides changes in the incidence of inorganic forces, there are equally continuous and still more involved changes in the incidence of forces which organisms exercise on one another. The plants and animals inhabiting each locality are held together in so entangled a web of relations that any considerable modification which one species undergoes acts indirectly on many other species, and eventually changes in some degree the circumstances of nearly all the rest. If an increase




of heat, or modification of soil, or decrease of humidity, cause a particular kind of plant either to thrive or dwindle, an unfavourable or favourable effect is wrought on all such competing kinds of plants as are not immediately influenced in the same way. The animals which eat the seeds or browse on the leaves of any of these plants are severally altered in their states of nutrition and in their numbers; and this change presently tells on various predatory animals and parasites.

There is no need of pursuing the subject into further detail. When the astronomic, geologic, meteorologic, and organic agencies that are at work on each species of organism are contemplated as becoming severally more complicated in themselves, and at the same time as co-operating in ways that are always more or less new, it will be seen that throughout all time there has been an exposure of organisms to endless successions of modifying causes, which gradually acquire an intricacy that is scarcely conceivable. Every kind of plant and animal may be regarded as for ever passing into a new environment-as perpetually having its relations to external circumstances altered, either by their changes with respect to it when it remains stationary, or by its changes with respect to them when it migrates, or by both.

Thus does Mr Spencer's doctrine that variations in an organism are caused by variations in its environmentby the conditions in which it is from moment to moment placed--give completeness to the theory of the evolution of species, and meet the objection that the initial variations are not accounted for. It could also be made to supply some answer to most other objections that have seemed to have any weight against Mr Darwin's

hypothesis, as, e.g., that many individuals must vary simultaneously if they are to perpetuate the variation ;1 that variation cannot have been always slow and minute; that it cannot have been fortuitous, but must have been in definite directions;3 that it is determined by inward tendencies; that it only oscillates about a normal line, and never transgresses the limits of species;5 and that it would require more time than astronomy can allow to geology. Climatal and other conditions, being nearly uniform over considerable areas, would cause similar variations in similar organisms; and being nearly uniform, or uniformly progressive through considerable periods, would keep the variations in definite lines; while the organism itself being a factor, there would be room to speak of innate tendency. When the changes were more sudden or more rapid than at present (as it is argued, by way of objection, that they have sometimes been') the variations might be more sudden and considerable-always supposing that the changes were not violent enough to kill the organisms-and this more rapid change at some periods would lessen the demand on geological time. The variations may do little more than oscillate when the conditions are nearly 1 North Brit. Rev., June 1867, p. 289. Mivart Genesis of Species, p. 271.

2 Edin. Rev., July 1871, pp. 202, 223, 227. North Brit. Rev. June 1867, p. 292. Reign of Law, p. 225. Genesis of Species, pp. 110, 117, &c.

3 Mivart: Gene

of Species, p. 143. Reign of Law, p. 273.

4 Prof. Owen: Anatomy and Physiology of Vertebrates. Mivart: Genesis of Species.

5 North Brit. Rev., June 1867, p. 281.

• North Brit. Rev., June 1867, p. 281. Sir W. Thomson: Trans. of Geol. Soc. of Glasgow, vol. iii. J. J. Murphy: Habit and Intelligence, i. 344. 7 Genesis of Species.



uniform, but the inevitable change in the cycle of external phenomena eventually carries the limits bodily forward, as the earth carries forward the lunar orbit and gives its apogee and perigee new absolute positions in space.

§3. The Evolution of Organic Matter.

The theory of Natural Selection derives Man from some lower animal, and derives all animals and plants from antecedent organisms greatly different from them, but does not take us back to the origin of living matter. Mr Darwin believes that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number, but considers the question of their firs: origin to be at present quite beyond the scope of science. He says that on the principle of Natural Selection with divergence of character, it does not seem incredible that both plants and animals may have been developed from some low intermediate form, such as the spore of an alga—and if we admit this, we must admit that all the organic beings which have ever lived on this earth may have descended from some one primordial form-but this inference is chiefly grounded on analogy, and analogy may be a deceitful guide.1

With regard to the origin of species, if we do not accept the theory of Natural Selection, or some other theory which derives one from another through the agency of natural law, we have to believe in the creation of each separate species out of nothing (which is inconceivable), or, in its being called into existence out of the inorganic elements. When we ask the origin of 1 Origin of Species, chap. xiv.

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