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Along the line of comparative physiology the rate of progress during the last 20 years has not been proportionate to the im- portance and attractiveness of this h'ne of investigation. There have been many individual researches of value, but there has been no adequate continuation of the broad and well-conceived work of Krukenberg 30 years ago. The difficulty is that few zoologists are expert in biochemistry, without which many paths of physiological research are impassable, and that few biochemists are aware of the many and pressing zoological problems that promise great results. It may be doubted whether in the whole range of zoology there is any kind of investigation more likely than comparative physiology to yield new knowl- edge of first-rate importance. Just as the discovery of the sig- nificance of the glands of internal secretion (suprarenals, thy- roid, etc.) has profoundly influenced the physiology of the vertebrates, so will a deepened physiology of the invertebrata modify the whole science.

Recent Advances. Although we are still awaiting concerted prosecution of comparative physiology, there have been numer- ous advances of great interest, of which two or three illustra- tions must suffice, (a) G. H. Parker's working-out of the various grades of nervous activity exhibited by a series of animals from sea-anemone upward is a good example of a physiological in- quiry of immediate interest in itself and also important in its bearing on the evolution of behaviour, (b) The continuation of the study of animal pigments may be illustrated by the work of Oscar Riddle, and not less interesting are such investigations as those of Gamble on the Aesop Prawn, of Sumner on the changeful patterns of flat-fish (see COLOURS OF ANIMALS), and of Minkiewicz on the apparent colour-disguises which some crabs find in the seaweed with which they mask themselves, (c) Also very promising is the inquiry (e.g. by Holmes and Schmidt) into the varied phenomena of " death-feigning." There is a prospect that it may be possible to arrange these in a series. Beginning with the sudden stoppage seen when the animal passes abruptly into a new medium we are led on to the spon- taneous catalepsy of some Phasmids, and thence to " animal hypnosis," and to the subtler forms of death-feint in various birds and mammals. On another series it may be possible to arrange a succession of physiological states, such as extreme fatigue, auto-intoxication, cold-coma, sleep, hibernation, and so on to latent life. Further study of latent life is desirable.

Hormones and Chalones. As was to be expected, the physio- logical discovery of the significance of the organs of internal secretion (ductless glands) has had its influence on zoology. The fact that stimulating hormones and quieting chalones produced by these ductless glands are distributed through the vertebrate body by the blood, and secure an integration hardly less important than that effected by the nervous system, has shed a flood of light on the functional correlation of parts. The idea of a regulative system is far-reaching, (a) It is of impor- tance in connexion with growth and development, as studies of the pituitary body show, (b) It sheds light on the ante-natal symbiosis characteristic of mammals, for it is known that hor- mones pass through the placenta not only from mother to off- spring, but from offspring to mother, to the great advantage of both. In such ways clues are being discovered which make it possible to understand how the uterus is prepared for the ovum, and how the milk is ready for the new-born mammal, (c) The whole dark subject of secondary sex-characters in animals has been illumined by the discovery of gonadial (testicular and ovarian) hormones which activate the development of the distinctive secondary characteristics of male and female. The internal secretions which pass from the ovary of a duck deter- mine the development of feminine characters and the inhibition of masculine characters. The latter normally remain latent, but if the ovary be removed the duck becomes a more or less perfect imitation of a drake of the same race; and this assump- tion of masculinity may affect behaviour as well as plumage (Goodale, 1916). (d) When acid gastric contents reach the duodenum there is produced in the intestinal wall a hormone

which the discoverers (Bayliss and Starling, 1902) called " se- cretin "; it is carried by the blood to the pancreas, where it provokes a greater production of digestive ferments which are carried by the pancreatic duct to the duodenum, there to dis- charge their appropriate function. It is for the zoologist to discover similar automatic regulative processes; and the quest for hormones in invertebrates, suggested by the work of J. F. Gaskell (1914) on the leech, is very attractive. Such an every- day incident as the cat's hairs " standing on end " when sur- prised by a dog admits of obvious interpretation in terms of the hormone of the suprarenal bodies.

Ecology. The old " natural history " cultivated a profitable field the study of the life of organisms in its interrelations, as it is lived in wild nature. Logically classified, it was an enquiry into " the higher physiology," that interplay of organisms, where account has to be taken of more than the internal economy of the individuals concerned. For if physiology be the study of the organism in its dynamic relations it must include the serious study of the intact creature in its natural surroundings, as one of a pair and of a family, as a member of an association and of a fauna. This ecology (or " bionomics ") is the oldest depart- ment of the science and it is indispensable; the question is how that of to-day differs from Reaumur's or Gilbert White's.

(a) Largely through Darwin's influence, there has been an increasing recognition of the complexity of interrelations amid which the organism h'ves and works. The recognition of the correlation of organisms has been not less profitable than the recognition of the correlations of organs within the body. It is not merely that, for completeness' sake, insects must be studied in relation to the flowers they visit, and the bird in relation to its mate and family, its " territory " and migrations there is yet more implied in the modern insistence on the concept of " the web of life." As Darwin indicated, it deepens our understanding of natural selection in the struggle for existence. " For in the gradually evolved and ever complexified system of interrelations there is a sieve of extraordinary delicacy, which discriminates between even minute variations to the plus or minus side " (Thomson, 1919, p. 95). Moreover, part of the difficulty of understanding the frequent and general progres- siveness of evolution (in the direction of increasing differentia- tion and integration) may be found in the gradual complexifying of the web of life, e.g. in the linkages between flowers and their insect-visitors. " There is established an external system of inter- relations which is always becoming more intricate, and this forms the sieve by which variations are sifted. There has been an evolution of sieves which partly accounts for the progressive evolution of the sifted " (Thomson, 1919, p. 96). The impor- tance of recognizing animate interrelations is familiar.

(6) Modern " natural history " has become frankly evolu- tionary. The " Souvenirs Entomologiques " of Henri Fabre must be taken as the crowning work of the older school; un- questionably an achievement of genius when we consider the patience, ingenuity, and intimacy of its observation, behind which lay an emotional sympathy, bringing, at times, even scientific reward, and yet an achievement robbed of its fruition by the observer's refusal to take Darwinism seriously, and by an unprofitable pessimism as to the possibility of any evolutionary interpretation whatsoever. A more progressive note is sounded in modern work permeated with the evolution-idea, such as Wheeler's studies on ants or Roubaud's on wasps.

(c) A third change, and a welcome one, is that " natural history" is becoming more analytic which will eventually mean a fresh synthesis. Thus the study of the migration of birds, for so long rather fumbling and anecdotal, has entered on a new phase of precision. This may be illustrated by Dr. Eagle Clarke's careful analysis (1912) of enormous masses of observa- tional material, the utilization of a new ringing method (Lands- borough Thomson, 1921), and by the precise experiments of Wat- son and Lashley (1915) on the homing of terns at the Tortugas.

Another illustration will be found in the still incipient en- deavpur of the shrewder observers to take advantage of the results of the modern study of animal behaviour. The older