and in the course of time accumulate them, so as to produce that great diversity of organic life, which we so highly admire." In the same way, if variations are in large measure definite, natural selection has not to sift out the serviceable from a large casual crop, but it still has its work to do.
The trend of recent zoology seems to be toward a confirmation of the central idea in Darwinism the selection of the relatively fitter variants in the struggle for existence. Thus there is no longer serious difficulty in meeting the criticism that slight initial variations could not have survival value, or that they would be swamped by intercrossing. There is also a deeper appreciation of the fact, on which Darwin so often laid emphasis, that selection is a manifold and often subtle process of sifting. Thus lethal selection must be distinguished from reproductive selection, and the operation of selective processes at many levels, even among germ-cells, must be recognized. The ubiquitous sifting is anything but automatic, since animals often select their environments, instead of being selected by them, and may be in various ways active agents in their own evolution. Natural selection operates in reference to an intricate web of life, and thus a nuance a shibboleth may have survival value. There has been an evolution of the sieves as well as of the sifted, for natural selection operates, generally speaking, in relation to an ecological Systema Naturae or system of inter- relationswhich has been increasingly elaborated through the ages; and this (along with what may be called organismal mo- mentum) is probably part of the explanation of the general progressiveness of organic evolution.
Sexual Selection. No part of Darwin's theory has met with a more critical reception than his theory of sexual selection; and yet it often has weathered the storm, (a) When there is forceful competition among rival males there is some evidence that the less well equipped with weapons will have fewer and less vigor- ous offspring, or it may be, none at all. This will operate like ordinary natural selection, (b) The same may be said of those masculine characters which aid males to find, pursue, and catch the female, (c) Sexual selection meant, for Darwin, all the processes of sifting that occur in connexion with mating and pairing, whether the female held the sieve or not; but he firmly believed that in a certain number of cases there is definite prefer- ential mating on the female's part, the more attractive males having a reproductive advantage. While Prof. T. H. Morgan has given, in his Evolution and Adaptation (1903), no fewer than 24 reasons for rejecting the theory of sexual selection, many zo- ologists believe that the reality of some measure of preferen- tial mating has been placed beyond doubt by the data furnished by Darwin himself (1871) and by the Peckhams (1889), Groos (1898), Cunningham (1900), Pycraft (1913), Julian Huxley (1914), Whitman (1919) and others. It is not necessary to suppose that the female chooses " the best of the bunch " from amongst her unequally endowed suitors; it will be enough if she remains unresponsive to the solicitations of certain of the less generously endowed, who do not arouse her emotional interest to the requisite pitch. This is in agreement with Darwin's own remark about the female bird: "it is not probable that she consciously deliberates: but she is most excited or attracted by the most beautiful, or melodious, or gallant males." It is not necessary to credit the female with a capacity for appreciating slight differences in decorativeness or musical talent or lithesomeness, or with a consistency in allowing these nuances to determine her preferential mating, season after season, generation after genera- tion. It is enough, as Lloyd Morgan says, that " the hen selects the mate which by his song or otherwise excites in greatest degree the mating impulse. Stripped of all its unnecessary aesthetic surplusage, the hypothesis of sexual selection suggests that the accepted mate is the one that most strongly evokes the pairing instinct." But this does not mean that the details do not count; it is rather that each is contributory to a general impression. Each has its effect; but synthetically, not analyti- cally. " Even when the female seems to choose some slight im- provement in colour or song or dance, the probability is that she is simply surrendering herself to the male whose ensemble
has most successfully excited her sexual interest " (Geddes and Thomson, Evolution, 191 1, p. 172).
It is interesting to inquire into the racial justification of the courtship habits often so prolonged, elaborate, and exhausting. According to Groos (1898, p. 242) the coyness of the female, which has to be overcome, is an advantageous check to the impetuous violence of the sex impulse. According to Julian Huxley the elaborate ritual of the Great Crested Grebe serves to forge an emotional bond. " The courtship ceremonies serve to keep the two birds of a pair together, and to keep them constant to one another." Pycraft (1913) also insists on the need for psychological interpretation. Karl Pearson suggests that pro- nounced and persistent preferential mating within a species being differentiated into types may lead to a physiological and psychological " isolation " (i.e. narrowing of the range of inter- crossing), and thus to " the relative or absolute mutual sterility of the differentiated types, i.e. to the origin of species " (Gram- mar of Science, 2nd ed. 1900, p. 418). Finally, it may be noted that the courtship habits must have played an important part in the evolution of sense (e.g. the aesthetic sense), and even of mind. This has been well illustrated by S. J. Holmes (1916) in connexion with the evolution of the voice, which began as a sex-call in amphibians, but gradually broadened out as parental summons, infantile cry, and kin-signal, until it became the mean of reasoned discourse. That after the last returns the first obvious in every serenade.
Naturalist Travellers. From Darwin's Voyage of the " Beagle ' (1844) and Wallace's " Travels on the Amazon " there has beer a fine succession of the tales of naturalist travellers. This ha been sustained in the last 20 years, and with increasing speciali- zation. In the older narratives there is naturally much that is not definitely zoological; in many of the newer the zoological or biological note is dominant. We may instance such books ; Alcock's Naturalist in the Indian Ocean, Hickson's Naturalis, in the Celebes, Hudson's Naturalist in La Plata, Saville-Kent's Naturalist in Australia, Semon's In the Australian Bush, and Siedlecki's Java.
Faunal Evolution. A line of zoological research which ha had some fine expressions in recent years is that of constructive faunistic interpretation, following the lead of Wallace in his Island Life (1880). One of the best examples is Dr. James Ritchie's Influence of Man on the Animal Life of Scotland (1920). Neolithic man penetrated into a country which, after the clean sweep of the Great Ice Age, had been restocked with anir life from the south of England and from the Continent. There were then no domesticated animals, nor aliens like rabbits and rats. But there were elk and reindeer, wild cattle, wild boar, perhaps wild horses a fauna of large animals on which lynx, brown bear, and wolves levied toll. This post-glacial fauna wa the capital with which prehistoric man started, to which he adde various imports from abroad, and which at times he likewis taxed heavily. With fine zoological scholarship Ritchie trace the changes brought about by domestication, by the destruction of beasts and birds of prey, by deliberate protection in variou interests, by the intentional and unintentional introduction of new animals from other countries, and by the cutting down of forests, the spread of cultivation, arid other human inter- ferences. While more species have been introduced into Britain than have been exterminated (the fauna having substantially gained in numerical strength), there has been a falling off in what may be called faunistic quality. For many masterly crea- tures have been replaced by elusive vagrants, and giants by pig- mies. " We have, in effect, lost more than we have gained; for how can the increase of rabbits and sparrows and earthwor and caterpillars, and the addition of millions of rats and cock- roaches and crickets and bugs, ever take the place of the fine creatures round the memories of which the glamour Scotland's past still plays the reindeer and the elk, the wolf, the brown bear, the lynx, and the beaver, the bustard, the crane, the bumbling bittern, and many another, lost or disappearing?" (p. 497). We have singled out this book as an outstanding instance of a kind of zoological investigation which is inter
