The respiratory mechanisms of the animal are necessarily in volved in such studies ; they may be dealt with in the form of the gross requirements of the animal over a period, or the detailed use of oxygen varying with such circumstances as exposure during an intertidal period may be determined.
The mode of transport of oxygen, its storage and the power that an animal possesses of incurring an oxygen debt, all form parts of such investigations. Indeed, the physiology of the chosen animal must be studied as a whole. When this has been done, not for one but for many animals, a science of comparative physiology will become possible; it will be parallel to comparative anatomy, based on evolution, and devoted to an understanding of the changes which have taken place in physiological activities, changes which must always have been such that the animals in which they were exhibited were workable wholes at every stage. Even with our present exceedingly scanty knowledge, the possibility of such a comparative physiology has become clear.
The chemical relationship which exists between many respira tory pigments, including haemoglobin, the colouring matter of red blood, and cytochrome, a respiratory pigment found in all cells of all plants and animals, points the way to many further investigations. But physiological investigations of a different kind are equally valuable. Cells, whether they compose the whole body of a Protozoan or are units in that of a Metazoan, have a physiology some parts of which can be determined. As cells come into contact with one another and with their surroundings only through their surface, it is evident that any knowledge of the nature and peculiarities of that surface will be of value in the solution of such problems as those presented by the process of cleavage and of histogenesis. The factors which are involved in the ordinary process of cell division, those which determine the position of the spindle and the movements of the chromosomes can clearly only be investigated by physiological methods.
An understanding of the mechanism which underlies the attain ment of a definite structure at the end of development involves chemical and physical researches. Only certain special parts of this story have so far proved attackable. Fertilization (q.v.), the most fundamental point in all Metazoan development, can be analysed. Its effects are twofold; it causes the first of a great series of cell divisions to take place, and it brings together in a single individual, qualities which have existed separately in two.
It is certain, from the morphological studies of cytologists and geneticists, that the second effect is brought about by the fusion of the cytoplasms and nuclei of the two gametes, and the resulting regainment of a diploid set of chromosomes. It might be sup
posed that the last occurrence was responsible for the initiation of cleavage, but it has been shown that it is possible to cause cleavage to begin, and even to continue to maturity, by a variety of chemical and physical treatments, and that the artificially par thenogenetic individuals so formed may retain a haploid num ber of chromosomes. (See PARTHENOGENESIS.) Thus the way is cleared for an investigation of the first effect in isolation; by physiological investigations it can be shown to involve a change in the character of the cell membrane which makes it permeable, and begins an active metabolism, and other changes in the cytoplasm which lead to the development of asters and a spindle.
Thus fertilization is multifold; some of its effects can be brought about by non-living agents, whilst others require a living cell. It will clearly be possible to investigate the mode of action of genes, and thus to gain a new line of attack on the problems of development. The most promising point for such an attack is on the determination of sex, which has been shown to be con trolled in normal cases by a balance between the actions of one special pair of chromosomes, and the remainder. But in a few cases the sex of an animal may be changed long after the nuclear character has been fixed at fertilization. The physiological mechanisms which are involved are still unknown, though clearly open to investigation.
Finally, the ductless glands, and especially the thyroid, have been shown to exercise a control over the development of an animal. Unless their secretions be present at the right time and in adequate amount, the whole process of development may stop or be diverted, the remainder of the mechanism failing to act.
Thus physiological investigations are those which seem likely to add most to the content of zoology in the future. They will, how ever, do so not in isolation, but when brought into contact with morphology and ecology and with a study of the natural conditions of life of the animals on which they are carried out. One particu lar group of such studies, those of "Animal behaviour" (see PSYCHOLOGY, COMPARATIVE) is of the greatest potential value, not only because its subject is of vital importance in all bionomics, but also because of its great influence on psychology.
It seems evident that the success of such animals as the spar row and starling, the rabbit and grey squirrel, which, introduced by man into new localities, have there developed into pests, is due as much to their "mental" characters, to morale, as to any special favourable qualities of their structure and general physiology.