Preservation of Favoured Races in the Struggle for Life;" and such a designation indicates with sufficient clearness that it was to "natural selection" that Mr. Darwin attributed the chief power in evolving new species through the modification of the old. Mr. Wallace accepts "natural selection" as a true factor, but he does not regard it as operating to the same extent in evolution as did Mr. Darwin. Other biologists, again, are inclined to adopt the idea that the evolution of living beings follows particular lines, along which the process is guided or directed partly by internal causes inherent in the constitution of the living being, and partly by external causes and by the surroundings of life. Concerning the relative importance of the various factors which biologists regard as of importance in determining the process of evolution, Huxley remarks that the exact place and power of "natural selection" "remains to be seen. Few can doubt that, if not the whole cause, it is a very important factor in that operation, and that it must play a great part in the sorting out of varieties into those which are transitory and those which are permanent. But," continues this high authority, "the causes and conditions of variation have yet to be thoroughly explored, and the importance of natural selection will not be impaired, even if further inquiries should prove that variability is definite, and is determined in certain directions rather than in others by conditions inherent in that which varies. It is quite conceivable that every species tends to produce varieties of a limited number and kind, and that one effect of natural selection is to favour the development of some of these, while it opposes the development of others along their predetermined lines of modification."

It forms no part of the purpose of this volume to discuss the merits of these varied views respecting the exact nature of the factors to which evolution owes its force and power. Perhaps any exhaustive account of this aspect of the subject is at present impossible with the materials at command. That which is infinitely more important in the first instance is the appreciation, firstly, of what evolution at large is and implies; and, secondly, of the proofs. and arguments on which the existence and operation of this process may legitimately be based. A brief statement of the Darwinian theory of evolution may, however, be given, inasmuch as this aspect. of the theory is that most frequently discussed and criticised both in scientific and in popular circles. It should be clearly borne in mind that the broad idea of evolution forms a foundation for every theory of the special fashion in which that process may be conceived to operate. "Darwinism" in this light is therefore to be regarded merely as one, but also as probably the strongest phase of those: speculative endeavours to show the "how" of living nature, just as evolution itself has supplied the answer to most of the biological "whys."

The term "natural selection," applied by Mr. Darwin to his theory of evolution, is in itself a highly expressive designation. It indicates an analogy with that process of "selection" whereby man chooses the animals he intends to breed from. As by human agency, the special features of any given race may be brought to the front in the progeny, or as other characteristics may similarly be obliterated by gradual changes in the appearance, size, colour, and structure of the animal and plant units, so it is contended an analogous principle that of "natural selection"—is traceable in the world around us. This process naturally tends to effect in nature the same or allied variations in species which man produces for a given end. In this view, natural selection is simply the natural result of a series of interactions between animal and plant life and its surroundings; and the gist of the process may be summed up in the statement that in the process of selection the weeded-out units die off, whilst the "selected" and stronger units, coming to the front, perpetuate their race, and thus tend, through their superiority and strength, to evolve new races and species.

It is an easy matter to summarise, in a series of propositions, the chief data upon which Mr. Darwin's theory rests. These propositions are as follows:

Firstly. Every species of animals and plants tends to vary to a greater or less degree from the specific type. No two individuals are alike in every respect; each inherits from its parents a general likeness or resemblance to the species, whilst it tends at the same time to diverge from the parental form.

Secondly. These variations are capable of being transmitted to offspring; in other words, by natural laws of inheritance, the variations of the parents appear in the progeny along with the natural characters of the species. This much is proved in the "artificial selection" by man, for breeding, of those animals whose characters it is desired should be transmitted to offspring.

Thirdly. More animals and plants are produced than can possibly survive. Each species tends to increase in geometrical progression, and all the individuals produced could not find food, or even surface-area whereon to dwell.

Fourthly. The world itself (i.e. the surroundings of animals and plants) is continually undergoing alteration and change, represented by climatal variation, the rising and sinking of land, &c.

Fifthly. There ensues a "struggle for existence" on the part of

living beings. Over-population means a struggle for food and for other conditions of life, such a consideration being really the doctrines of "Parson Malthus" applied to the animal and plant worlds at large. Hence it follows that as some forms will be better adapted (by variation) than others to their surroundings, the former will come to the front in the struggle. Nature, so to speak, will "select" those individuals which best adapt themselves to their surroundings, and will leave the rest to perish. This is the "survival of the fittest." The change of surroundings, already postulated, will further induce and perpetuate variations in those individuals which survive.

Sixthly. A premium is thus set by nature upon variation, inasmuch as the varying and surviving individuals will transmit their peculiarities to their offspring.

Seventhly. Thus "varieties" of a species are first produced; the "varieties" becoming permanent, form "races;" and the "races," in time, differ so markedly from the original species whence they were derived, as to constitute new "species." Eighthly. Past time has been, to all intents and purposes, infinite. Hence it is probable that the existent species of animals and plants have been evolved (through "natural selection," acting through long periods of time) from a few primitive and simple forms of life, or possibly from one such form alone.

Such is a summarised statement of Mr. Darwin's views. His theory of "Sexual Selection" may be viewed as supplementary to that of natural selection, and as serving likewise to account for certain phenomena of which the former takes little heed. The process of sexual selection is that whereby the males of many species secure the females after contests. The result of these contests is that the stronger and victorious males will transmit to their offspring any peculiarities of form or constitution which they themselves possess, and in virtue of which they became victors over others. In this way variation is again seen to be favoured. Then, secondly, the "selection" of a mate is often determined, not by the males, but by the females. In such a case it is assumed that those males which exhibit (as seen typically amongst birds) special features in the way of colour, plumage, size, or ornamentation, will be preferred and chosen. Variations are thus once more produced; since the special characters of the male will be reproduced in the offspring, whilst the perpetuated accumulation of such characters will in due time modify the species and evolve new races therefrom. By aid of the theory of

"sexual selection" Mr. Darwin accounts for many of the special features and possessions of animal races. Thus, the song of birds, the brilliant plumage and colours of many species, and the curious and peculiar ornamentation of many forms, altogether inexplicable on any ordinary theory of utility, are seen to be useful or necessary adjuncts, on the theory of "sexual selection," to the modification of species and to the evolution of new races.

The foregoing statement of the Darwinian theory will enable the reader to follow with greater advantage the arguments and illustrations adduced in the succeeding chapters in support of the evolution theory at large. It only remains in the present instance to indicate the order and succession in which the evidences of evolution are herein presented.

An account of the methods in which the study of modern biology or natural history is carried out, forms the subject of the second chapter. Such an account will serve to place the reader in possession of the chief data, from a knowledge of which the naturalist is enabled to construct a reasonable and harmonious series of details respecting the living denizens of the globe. The special inquiries of the biologist are duly noted, and the divisions of biology which supply answers to the pertinent queries of the scientific investigator are also detailed. Incidentally, the bearings of ordinary biological details on evolution are also discussed, and a suitable introduction is thus afforded for succeeding studies.

In the next and third chapter, the reader is made acquainted with the constitution of the animal and plant worlds. The knowledge of the general relationship of animals and of plants to each other, viewed in groups and as individuals, forms a necessary foundation for all biological studies, whether viewed in reference to the theory of evolution, or merely as a part of ordinary information respecting the universe of life as a whole. In this chapter, the bearings of the constitution of the animal and plant kingdoms on the theory of descent are duly detailed; and a sketch of the primary classifications of animals and plants is also included in the general history of the worlds of life.

The fourth chapter introduces the subject of "protoplasm." On the due appreciation of the relations of this substance as the "physical basis of life" to the constitution of the living body, rests the clear understanding of many fundamental points in connection with animal development. Similarly, the inferences which the evolutionist is led to draw from the universality of protoplasm as the common material of living beings, are only appreciable when the nature of this curious and all-pervading substance is set forth in detail. No step is possible in biological advance until the facts relating to protoplasm and its relations to life are mastered; and in

the discussion of such a topic certain fundamental truths and propositions of biology therefore fall to be discussed.

Thus fortified and prepared by these introductory details, the evidences of evolution as the great process which summarises in itself the forces and tendencies of living beings fall to be noted. The first of these evidences is constituted by "rudimentary organs," and the tale they tell of animal and plant modification. Here the curious nature of these apparently useless parts is seen to be fully borne out by the idea that they refer "to a former state of things," and that they represent the natural, but deteriorated and vanishing remains of structures once useful in the ancestors of the animals that now possess them.

The sixth chapter strikes a somewhat related key-note to that touched in the preceding section. The evidence deducible from the modifications which animal structures have undergone is largely in favour of evolution. The structures specially selected for treatment in this chapter are the tails, limbs, and lungs. It is attempted to be shown that these organs illustrate in the clearest manner how adaptation to new ways of life is induced by alterations in the habits and surroundings of animal forms. Incidentally, information is likewise afforded respecting certain interesting aspects of the structure of higher animals.

The science of likenesses (or homology) forms the special topic of the succeeding section. Herein the general deductions of "homology" are discussed and illustrated from both animal and vegetable worlds. The broad likenesses between animals which were discussed in the third chapter, are here specialised, and the natural correspondence existing between parts and organs, often of the most diverse appearance, is duly dwelt upon. In its general tenor, this chapter will be found to follow out the line of argument specially selected in chapter sixth.

The subject of "missing links" is treated in the eighth chapter. No topic in all the wide range of evolution demands more detailed treatment than that of the "links" between apparently distinct groups of animals the existence of which the theory itself postulates, and the necessity for which is a matter of popular notoriety. The higher animals have been specially selected for treatment in this chapter, not merely because the case for evolution is more likely to be duly appreciated when these forms are selected for discussion, but because the evidence is overwhelmingly clear in favour of evolution when the higher groups are examined, and also because links in lower life are duly treated in succeeding chapters under the head of "Development."

The succeeding three chapters deal with the evidence afforded by development in favour of evolution. All evolutionists may