formed by the individuating principle of the creature to be nourished. At the very same instant the matter composing it has become part of the actual matter of that creature's body. This view accords well with the facts of biology. The nutrition of the body by food is a necessary condition for growth, and growth of the body, as we have long ago seen,* is a sort of generation. Closely connected with growth, also, are those processes of repair after injury,† to which attention was before directed; to the balanced harmonies of such processes of repair the phenomena of lateral and serial homology,‡ normal and teratological,§ are evidently akin, together with the symmetrical bodily modifications which occur in various diseased conditions. All these various processes can be well accounted for by the activity of the principle of individuation animating each individual organism, and they can be satisfactorily accounted for in no other way. nesis of Of all these processes nothing is more wonderful than Psychoge the development of the individual. In all animals above individual. the Protozoa || we find that a single cell gives rise first to others like it, which arrange themselves in a definitely symmetrical manner, and afterwards give rise to others which are unlike their predecessors. Thus a variety of different kinds of cells-different tissue-clements: connective, cartilaginous, osseous, sanguineous, muscular, nervous, etc. are generated in definite order, according to an innate preordained law, and these again divide and coalesce in diverse ways, and so by degrees build up the different organs of the developing body in each of which these tissues enter in various definite degrees and proportions. About the whole process there is nothing which appears haphazard or fortuitous, but all proceeds with order and precise regularity. The process of development of each organism is thus the embodiment of a directing idea of which it has no perception itself. But no mere idea can by itself direct anything or even exist. It must pertain to some active principle which possesses it, and which may, if conscious, be guided by it, and if unconscious may make See above, p. 152. See above, p. 320. See above, p. 163. † See above, p. 169. Phyllogenetic hypotheses. manifest the idea, pertaining to a higher intelligence, which it unconsciously embodies. Such is the principle which animates and individualizes every organism. The presence of such an innate, directing constitutive element is, indeed, especially evident in the development of the embryo, where all that takes place is manifestly controlled and directed to the effecting of one definite end-even more plainly, perhaps, than in the healing of a wound or the reproduction of a lost limb. It is manifestly a process. carried out in fulfilment of a predetermined end, and is an instance of "final causation." Any one who would pretend that the mere conflict of independent efficient causes can produce and co-ordinate whole series of effects, resulting in the attainment of a definite end which they have all concurred to produce, would certainly go against all our experience. Any one, also, who should pretend that we cannot affirm a "purpose" to exist in different natural processes (ie. who denies that we can assert a "final cause" for any phenomenon) because we may be unable to state the final cause of the whole series of physical phenomena, would be like a soldier who, because he was ignorant of the plan of campaign of his commander-in-chief, should pretend that therefore he could not infer that commander's purpose in sending medical stores to the military hospital. Ontogeny is, therefore, a teleological process, and it is one carried out through the action of the directing and innate energy of the developing animal-its principle of individuation, or psyche, by which, as before said,* the living organism builds up its being. It is thus a process of psychogenesis.† Having thus pondered over the life-processes of the individual organism, and armed ourselves with the foregoing conception of the nature of each, let us turn to consider the question of the genesis of new species. We may conceive the evolution of new specific forms to have been brought about in one or other of the six following ways: 1. Entirely by the action of surrounding agencies on organisms having a merely passive capacity for being * See above, p. 436. † See above, p. 440. indefinitely varied in all directions, and with no positive. inherent tendency to vary, whether definitely or indefinitely. 2. Entirely by innate tendencies in each organism to vary in certain definite directions. 3. Partly by an innate tendency to vary indefinitely in all directions, and partly by the limiting actions of surrounding agencies checking all variations save those which happen accidentally to be favourable to the organisms which vary. 4. Partly by an innate tendency to vary indefinitely in all directions, and partly to external influences which not only limit, but stimulate and promote variation. 5. Partly by an innate tendency in organisms to vary definitely in certain directions, and partly to external influences acting on variation only in the way of restriction and limitation. 6. Partly by an innate tendency to vary definitely in certain directions, and partly to external influences which, in some respects, act restrictively, and in other respects act as a stimulus to variation. The best known and most popular explanation is that given by Mr. Darwin in his "Origin of Species." According to this view new species have been brought about by the action of what he called "natural selection;" that is, by the accidental preservation, through the chance action of the destructive powers of nature upon minute fortuitous variations, occurring in all directions, of such variations as happened to be the most favourable. His hypothesis, therefore, accords with the third of those above suggested as possible explanations. So many more living creatures being generated than could live, he deemed such chance destructive agencies to be sufficient reason for the formation of new species, since only those which presented favourable variations could possibly survive such destructive action. He thus sought to account for all the forms and powers present in nature, including the mind of man with all its perceptions of truth, goodness, and beauty. We have been careful to provide the reader with references to those facts which favour Mr. Darwin's hypothesis. Such are the various interrelations which exist between Heredity and environment. organisms and the phenomena of mimicry.† We have also pointed out how favourable variations in voice, colour, other ornamentation, or strength, may enable an animal to survive, and to transmit such characters to offspring by obtaining a mate. Let us now look a little closely into the nature of the possible factors directly affecting the evolution of species by natural generation. The first and most potent of these is the action of heredity.§ Heredity is obviously a property, not of new individuals, not of offspring, but of parental forms. It is the innate tendency which each organism possesses to reproduce its like. If, as we have before pointed out, it acted alone, each parent organism would produce offspring completely like itself. This fundamental biological law of reproduction may be compared, as it has been compared by Dr. Gasquet, with the physical first law of motion, according to which any body in motion will continue to move on uniformly at the same rate and in the same direction until some other force or motion is impressed upon it. The fact that new individual organisms arise from both a paternal and a maternal influence, and from a line of ancestors, every one of which had a similar twofold origin, modifies this first law of heredity only so far as to produce a more or less complex compound of hereditary tendencies ¶ in every individual, the effect of which must be analogous to the mechanical law of the composition of forces. All such action is but "heredity" acting in one or other mode; but there is another and fundamentally different action which has to be considered, and that is the action of the environment upon nascent organisms-an action exercised directly upon them, or indirectly upon them through its direct action upon their parents. The action of external conditions in many instances we have already noted.** It is, however, obvious that the very same external influences will produce different effects on different species, as also that the nature of some species is more stubborn See pp. 369-371. § See above, p. 173. † See pp. 372, 373. See above, p. 440. See p. 374. See above, pp. 172, 174. and less prone to variation than that of 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. Thus both the amount and the kind of variability differ in different races, and such constitutional capacities, or incapacities, tend to be inherited by their derivative forms, and so every kind of animal must have its own inherent powers of modifiability. or resistance. Therefore no organism or race of organisms can vary in an absolutely indefinite manner; and if so, then unlimited variability must be a thing absolutely impossible. The foregoing considerations tend to show that every variation is a function of "heredity” and “external influence"-i.e. is the result of the reaction of the special nature of each organism upon the stimuli of its environ ment. In addition to the action of heredity and the action of the environment, we have also recognized another kind of action manifesting the existence of an internal force to which are due so many interesting cases of serial and lateral homology to which we have just referred, and which cannot be due to descent. If we not only assume that vertebrate animals are descendants of a common ancestor, but also make the extravagant (and, as we believe, untrue and unwarrantable) assumption that they are the descendants of a creature formed by the coalescence in a linear series of independent, separate, lowly animals, it only makes the remarkable facts of lateral homology still more mysterious. On this assumption, the genetic relationship between a man's arm and a bird's wing must be almost infinitely closer than that between a man's arm and his leg, and this again much more recent than that between his right and his left arm. According to such an hypo It has been suggested that all kinds of worms have arisen from ancestors each of which was like one of the segments only of the much segmented worm which has descended from it. Such simple ancestor has been supposed to have had the habit of multiplying by spontaneous fission, and some of its descendants to have been benefited (and therefore preserved) by having divided only imperfectly, and so given rise to creatures (such as the Annelids and Arthropods) the bodies of which consist each of a longitudinal series of segments. Vertebrate animals being also supposed to have descended from some kind of worm, the successive portions of the body would thus originally have had separate ancestors, the descendants of which had found their profit in not dividing completely. |