COMPARATIVE PHYSIOLOGY. Physiology as defined in the medical curriculum of most institutions of higher learning confines its enquiries pre-eminently to man and his nearest allies, focussing attention on those activities of the body which are of special interest to the problems of health and disease and those which can be conveniently studied within a narrow range of animal material. From a practical point of view this preoccupa tion with clinical objectives has its drawbacks: for medicine has been repeatedly enriched by the more catholic approach which makes no restriction on its materials or problems. This more embracing attitude to the scope of physiological science is referred to by the terms general physiology and comparative physiology. In practice there is no hard and fast distinction between these two terms, though some authors have endeavoured to define one.
Modern comparative physiology represents a rapprochement between the teaching of physiology and zoology which is really a return to the practice of an earlier period, when morphology and physiology were not as yet divorced. Many of the zoologists of the earlier half of the 19th century regarded morphology as the handmaid of physiology. To men like Milne-Edwards it is clear that the ultimate problems of the biologist were physiological. With the rising influence of the evolutionary theory the purely architectural approach of the Cuvierian school received a strong impetus. Structural details of little or no significance as aiding an understanding of the properties of living matter came to be cherished for their historical interest as a possible means of throw ing light on the ancestry of man and the pageant of life in pre ceding geological epochs. The loth century has witnessed a strong reaction against this tendency. This is partly because pro gress in palaeontology and in the morphology of recent animals has made it clear that the extent of parallel and convergent evolution presents unsurmountable difficulties to the task of con structing a classification of living beings which expresses in all its minutiae their family relationships. It has arisen also through the recognition that the ultimate justification of the evolutionary hypothesis is bound up with the understanding of the machinery of heredity and variation. The awakened interest in experimental problems which has resulted from Mendelian research has exerted its influence over the whole field of zoology, so that morphology is taking second place in current investigation. Morphology, the study of structure as a separate issue, has completed its task. It has placed the principle of geological succession on a firm foundation. It has laid down as far as it is possible, at least for some time to come, those major associations in systematic zoology which can with any legitimate confidence be considered to indicate ancestral relationships. Further than that it cannot progress to any considerable distance except in so far as fresh relics of extinct animals may be unearthed from time to time. Here and there lacunae remain. Certain practical problems in relation to parasitic forms of life justifiably continue to absorb a considerable measure of attention. But there is no likelihood that morphology divorced from physiology will contribute to the building up of another generalization of the magnitude of the evolutionary hypothesis. In short the study of the lower organisms is likely to receive its chief driving force from comparative physiology.
The practical justification for extending the scope of physio logical science to the study of the lower animals resides partly in the fact that many phenomena of universal biological significance can be more conveniently studied in species only remotely related to man ; partly also because it is sometimes easier to approach the latter with the detachment and objectivity which are essen tials of scientific method. Thus, on the one hand, the phenomena of reproduction in its most fundamental aspects and the physical properties of the cell as a physiological unit present almost un surmountable difficulties in the case of viviparous warm-blooded animals like man and his nearer relatives : but rapid progress can be made in dealing with simpler aquatic forms of life which spawn their reproductive cells into the sea; and such progress suggests methods which may direct fruitful enquiry into the investigation of more difficult material. Again, it is at present impossible to think about human behaviour in purely objective terms; yet con siderable progress has been made in the acquisition of predict able knowledge about behaviour in more lowly organisms, and from such enquiries have emerged habits of treatment which are paving the way to a scientific study of behaviour in more com plex beings. One might say that the business of the comparative physiologist is to find the right animal for the solution of a par ticular problem. In no field is the value of the comparative method emphasized in a more striking manner than in the experi mental study of inheritance, which, rightly conceived, is an essen tial compartment of physiological science, though frequently dis cussed under the separate heading Genetics (q.v.). For rapid advance in genetics, it is necessary to deal with organisms that breed prolifically, attain sexual maturity in the least possible time, and present well-defined true-breeding varieties for study. The fruit-fly Drosophila melanogaster passes through its lif e cycle in a few days, breeding prolifically and has a great number of discrete varieties which breed true to type (see CYTOLOGY). It would hardly be an exaggeration to say that more could be found out in a year's research on inheritance in Drosophila than in several thousand years of study of human material. Yet many of the generalizations derived from the study of the fruit-fly and other low organisms apply to the problems of inheritance in man himself, though the principles themselves would never have been elucidated if no other forms had been accessible.
Within the limits of space available it is only possible to select a few lines of enquiry in which the comparative method has proved especially illuminating. First and perhaps foremost the physiology of the fertilization and early development of the egg may be mentioned. The reader who is not familiar with the nature of the fertilization process is referred to the article CYTOLOGY. From the physiological standpoint the problem of absorbing in terest is the material nature of the stimulus imparted by the sperm to the egg, by which a phase of active cell-division is initiated in the latter, culminating in the production of a new individual. A new epoch dawned in the closing year of the 19th century, when Loeb, perhaps the most distinguished pioneer of the comparative method in physiology, found it possible by means of purely physicochemical stimuli to induce eggs of the sea-urchin to develop in a normal manner without contact with the sperm or male element. ' By placing the eggs in certain solutions of organic acids and subsequently treating them with concentrated sea-water, the content of which in dissolved material had been raised by adding such substances as sugar or salts, eggs of the sea urchin can be induced to segment and to grow into larvae, though free of any contamination by sperm. This discovery initiated a large volume of research, the net result of which has been to show that the physical changes produced by these artificial agencies are similar to the internal events that precede active development in normally fertilized eggs: and it has now been possible to produce fertilization by physicochemical means in eggs of species of most groups of cold-blooded animals whose eggs are fertilized exter nally. Modern work on tissue culture (q.v.) encourages the antici pation that this may be achieved in the near future in warm blooded animals, possibly ultimately in man himself. A fuller exposition of this work would lead into technical issues, but one feature of the development of the sea-urchin's eggs discovered by Loeb is of the utmost interest, namely the immense increase of respiratory activity that accompanies fertilization. Warburg found that the oxidation process was proportional to the iron con tent of the egg, and that by carefully drying the egg-substance a powder might be prepared that would continue to absorb oxygen and burn up organic compounds, as does the living cell. Not only did he separate the functions of respiration from what would be ordinarily called "living matter," but succeeded in making sus Pensions of charcoal containing iron capable of facilitating the spontaneous oxidation of organic substances such as occur in animal food, and these physical respiratory models were shown to be affected by various types of reagents which reduce oxygen consumption in the living body in a precisely analogous manner.
In the study of behaviour from the physiological standpoint the progress that has been achieved illustrates in a convincing manner the intellectual discipline to be derived from dealing with animals that can be approached readily without the bias towards the tra ditional anthropomorphic attitude that is inevitable in dealing with man's nearer relatives. Thus the analysis of behaviour of jelly-fishes and sea-anemones by Romanes, Parker and others, can be discussed without difficulty in terms of the properties of the conducting (nervous) tissues without recourse to terms like memory or will, like or dislike. In the behaviour of the more complex animals notable advance has been made by studying bodily orientation in relation to group stimuli such as light and gravitation. Thanks to the labours of Loeb, Mast and others the old meaningless statement that the moth moves towards the candle because it likes the light, has given place to an understanding of how light stimulating certain parts of the retina discharges nervous impulses to particular groups of muscles whose contraction brings the body into such a position that it must move along the path of the incident rays. The new interpretation makes possible pre dictions that are actually verifiable, as for instance the fact that, if one eye is blackened, the moth will fly in circles. The applica tion of the comparative method to the study of animal behaviour has been one of the main contributing influences to the develop ment of the modern school of behaviourist psychology (see