802 ZOOLOGY cepted as the central, all-embracing doctrine of zoological and botanical science. Darwin succeeded in establishing the doctrine of organic evolution by the introduction into the Aveb of the zoological and botanical sciences of a new science. The subject- matter of this new science, or branch of biological science, had been neglected : it did not form part of the studies of the collector and systematist, nor was it a branch of anatomy, nor of the physiology pursued by medical men, nor again was it included in the field of microscopy and the cell-theory. The area of biological knowledge which Darwin was the first to subject to scientific method and to render, as it were, contributory to the great stream formed by the union of the various branches, the outlines of which we have already traced, is that which relates to the breeding of animals and plants, their congenital varia tions, and the transmission and perpetuation of those variations. This branch of biological science may be called thremmatology (0/ae/A/m, "a thing bred"). Outside the scientific world an immense mass of observation and experi ment had grown up in relation to this subject. From the earliest times the shepherd, the farmer, the horticulturist, and the " fancier " had for practical purposes made them selves acquainted with a number of biological laws, and successfully applied them without exciting more than an occasional notice from the academic students of biology. It is one of Darwin s great merits to have made use of these observations and to have formulated their results to a large extent as the laws of variation and heredity. As the breeder selects a congenital variation which suits his requirements, and by breeding from the animals (or plants) exhibiting that variation obtains a new breed specially characterized by that variation, so in nature is there a selection amongst all the congenital variations of each generation of a species. This selection depends on the fact that more young are born than the natural provision of food will support. In consequence of this excess of births there is a struggle for existence and a survival of the fittest, and consequently an ever-present necessarily- acting selection, which either maintains accurately the form of the species from generation to generation or leads to its modification in correspondence with changes in the sur rounding circumstances which have relation to its fitness for success in the struggle for life. Darwin s introduction of thremmatology into the domain of scientific biology was accompanied by a new and special development of a branch of study which had previously been known as teleology, the study of the adaptation of organic structures to the service of the organisms in which they occur. It cannot be said that previously to Darwin there had been any very profound study of teleology, but it had been the delight of a certain type of mind that of the lovers of nature or naturalists par excellence, as they were sometimes termed to watch the habits of living animals and plants, and to point out the remarkable ways in which the structure of each variety of organic life was adapted to the special circumstances of life of the variety or species. The astonishing colours and grotesque forms of some animals and plants which the museum zoologists gravely described without comment were shown by these observers of living nature to have their significance in the economy of the organism possessing them ; and a general doctrine was recognized, to the effect that no part or struc ture of an organism is without definite use and adaptation, being designed by the Creator for the benefit of the creature to which it belongs, or else for the benefit, amusement, or instruction of his highest creature man. Teleology in this form of the doctrine of design was never very deeply rooted amongst scientific anatomists and systematists. It was considered permissible to speculate somewhat vaguely on the subject of the utility of this or that startling variety of structure ; but few attempts, though some of great importance, were made to systematically explain by observa tion and experiment the adaptation of organic structures to particular purposes in the case of the lower animals and plants. Teleology had, however, an important part in the development of what is called physiology, viz., the knowledge of the mechanism, the physical and chemical properties, of the parts of the body of man and the higher animals allied to him. The doctrine of organs and func tions the organ designed so as to execute the function, and the whole system of organs and functions building up a complex mechanism, the complete animal or plant was teleological in origin (see PHYSIOLOGY), and led to brilliant discoveries in the hands of the physiologists of the last and the preceding century. As applied to lower and more obscure forms of life, teleology presented almost insur mountable difficulties; and consequently, in place of exact experiment and demonstration, the most reckless though ingenious assumptions were made as to the utility of the parts and organs of lower animals, which tended to bring so-called comparative physiology and teleology generally into disrepute. Darwin s theory had as one of its results the reformation and rehabilitation of teleology. Accord ing to that theory, every organ, every part, colour, and peculiarity of an organism, must either be of benefit to that organism itself or have been so to its ancestors : no peculiarity of structure or general conformation, no habit or instinct in any organism, can be supposed to exist for the benefit or amusement of another organism, not even for the delectation of man himself. Necessarily, accord ing to the theory of natural selection, structures either are present because they are selected as useful or because they are still inherited from ancestors to whom they were useful, though no longer useful to the existing representa tives of those ancestors. The conception thus put forward entirely re-founded tele ology. Structures previously inexplicable were explained as survivals from a past age, no longer useful though once of value. Every variety of form and colour was urgently and absolutely called upon to produce its title to existence either as an active useful agent or as a survival. Darwin himself spent a large part of the later years of his life in thus extending the new teleology. A beginning only has as yet been made in the new life of that branch of zoological and botanical study. The old doctrine of types, which was used by the philo sophically-minded zoologists (and botanists) of the first half of the century as a ready means of explaining the failures and difficulties of the doctrine of design, fell into its proper place under the new dispensation. The adher ence to type, the favourite conception of the transcendental morphologist, was seen to be nothing more than the ex pression of one of the laws of thremmatology, the persist ence of hereditary transmission of ancestral characters, even when they have ceased to be significant or valuable in the struggle for existence, whilst the so-called evidences of design which was supposed to modify the limitations of types assigned to Himself by the Creator were seen to be adaptations due to the selection and intensification by selective breeding of fortuitous congenital variations, which happened to prove more useful than the many thousand other variations which did not survive in the struggle for existence. Thus not only did Darwin s theory give a new basis to Effects of the study of organic structure, but, whilst rendering the Darwin s general theory of organic evolution equally acceptable and u ^ ) y necessary, it explained the existence of low and simple forms Z0 ology. of life as survivals of the earliest ancestry of more highly
complex forms, and revealed the classifications of the