naturalists read the man into the beast without let or hindrance or else reacted to the other extreme of regarding animals as automata. It is now recognized as desirable that an attempt be made to describe behaviour with reference to definite categories, of intelligence, instinct, individual habitation, association, tropisms, reflexes, and the like. To mention what may seem at first sight a small matter, Miss Frances Pitt (1920), in study- ing the well-known method the thrush adopts of breaking snails' shells on a stone-anvil in the wood, takes the trouble to experiment with a young thrush to see whether the behaviour is instinctive or whether it has to be learned. Her account of the bird's gradual learning, by a " trial and error " method, is of real scientific value.
Luminescence. There is definite progress to record in the study of many animal activities which stand somewhat apart from the everyday life of the body. The inquiry into the nature of animal luminescence is a case in point (see Harvey, 1920). Luminescence is known to occur in no fewer than 36 orders of animals; it is a chemical phenomenon that may manifest itself after life has ceased (bacteria apart); it may be produced in situ in the cells in which the photogenic substance is produced, or there may be a luminous secretion that exudes over the surface of the body and forms a glimmering trail hi the sea or on the ground; it may have its seat in more or less elaborate luminous organs which often show a remarkable convergence to eyes (the chemiphotic approaching the photochemical). The production of the animal light may be continuous or it may be periodic and even rhythmic. In " fire-flies " the light consists wholly of visible rays, with no infra-red or ultra-violet, and none of the energy is lost in the form of heat. It is practically perfect " cold light." The matter has been probed furthest in Lampyrid beetles, in the small crustacean Cypridina, and in the boring bivalve Pholas, and the result has been to confirm the contention of Raphael Dubois (1887) that a protein substance, luciferase, acts, in the presence of oxygen and water, on another substance, luciferin, which has much in common with peptones. The luciferins of different luminescent animals are different, but all luciferins have a good deal in common, and it is the same for the luciferases. It is probable that luciferase is an organic enzyme or catalyst which oxidizes luciferin, or accelerates its oxidation, with the result that light is produced. It is interesting that the chemical physiology of animal light has outrun its ecologic interpretation. In some cases the light may be no more than the byplay of some physiologically important chemical change, but a biological interpretation is demanded when there is an elaborate luminous organ or a definite arrangement of organs. Unfortunately, however, the interpretations suggested remain more or less of a speculative nature. The light may scare away intruders; it may be a lure attracting booty; it may illumine the surroundings; it may facilitate the recognition of kin; and it may serve as a sex-signal in mating. What seems a physiological byplay has thus been utilized and elaborated in quite different directions.
Tropisms. On the inclined plane of animal behaviour tro- pisms occupy a place somewhat above ordinary reflex actions, and it is certain that an understanding of the ways of animals is impossible unless the role of tropisms is duly appreciated. This need not involve an attempt to coerce behaviour of a higher order (e.g. implying intelligent control) into categories mani- festly too small an exaggeration which sometimes seems to attend the brilliant experimental work of Jacques Loeb (1918). Yet tropisms must be taken account of, and Loeb has shown that they are obligatory or forced movements of the animal as a whole, which more or less automatically secure physiological equilibrium in relation to outside stimuli, such as light or heat, gravity or electricity, diffusing chemicals or water currents. When a moth, constitutionally adapted to nocturnal activity, but positively heliotropic none the less, comes in its flight within the sphere of influence of a lit candle, and has one eye much more illumined than the other, owing to the direction in which it happens to be flying, more intense chemical processes are set up hi the more illumined eye. This means that on the illumined
side there is a relative increase in the mass of certain chemical products. But stimulations are always passing from the brain of the flying moth to the contracting muscles, and if the physio- logical symmetry of the brain has been disturbed by the unequal illumination of the eyes, the muscles on the more illumined side are thrown into a state of stronger tonus, with the result that they respond more forcibly to nervous stimulation, and therefore turn the head and body of the moth directly toward the candle near which it is flying. " As soon as the plane of symmetry goes through the source of light, both eyes receive again equal illumi- nation, the tension (or tonus) of symmetrical muscles becomes equal again, and the impulses for locomotion will now produce equal activity in the symmetrical muscles. As a consequence, the animal will move in a straight line to the source of light until some other asymmetrical disturbance once more changes the direction of motion " (Loeb, 1918, p. 14). When elvers go straight up stream against the current, when small crustaceans crowd to the shaded or to the warmer corner of the aquarium, when newly hatched turtles make for the sea, we have to do with tropisms. In many cases, however, even among the lower animals a higher note seems struck. For Jennings (1906) there is trial after trial of different movements, and there is a selection of that which brings relative satisfaction; an interpretation to which this observer adheres, despite criticism from Loeb.
Zoological Parasitology. Progress in the study of parasites has been a very marked characteristic of the zoological work of recent years. This is congruent with the increased attention that' has been given to the broad fact of the inter-relations between organisms, and details of the important medical results achieved will be found in the separate medical articles on this or that form of parasitic disease (e.g. MALARIA, SLEEPING-SICKNESS, BILHARZIOSIS, etc.). The tracking of the life-history of parasites is but a particular instance of this kind of investigation, with a special interest when the parasites affect Man and his domesti- cated animals and cultivated plants. Good examples are not far to seek the malaria organism (plasmodium), and the mos- quito's share in its development and dissemination; the try- panosomes that cause sleeping-sickness and other diseases, and the tsetse flies and other insects implicated in their transmission; the hookworms (Ankylostoma and Necator) which depress the vitality of enormous numbers of the inhabitants of tropical and sub-tropical countries; and the species of the formidable Bil- harzia (Schistosomum) whose life-history, discovered by Dr. Leiper, is bound up with freshwater snails. Apart altogether from the immense practical importance of these investigations, they are full of theoretical interest. Thus, to take the two kinds of Bilharzia in Egypt, one species, Schistosomum mansoni, has ova with a lateral spine, is chiefly associated with the snail Planorbis, and produces intestinal bilharziosis; while the other species, S. Haemalobium, has ova with a terminal spine, is chiefly associated with the snail Bulinus, and produces renal bilharziosis obviously a fine instance of specificity. As several human diseases have now yielded to attack from the zoological side, it is not over-sanguine to consider whether there may not be a much-desired clue in the fact that cancer of the stomach of the rat is produced by a nematode whose carrying host is the American cockroach. Very interesting and somewhat surprising is the discovery (see Rennie, White, and Harvey, 1921) that the so-called " Isle of Wight disease " in hive-bees, which spread in a few years all through Britain, is causally connected with the presence of a minute mite, Tarsonemus tvoodi, in the cavity of two anterior tracheas. More familiar is the demon- stration of the fact that true pearls may have a parasitic origin.
(C) EMBRYOLOGICAL ZOOLOGY
While descriptive morphological embryology continues on lines which require no vindication we have only to think of recent work on the development of the marsupials and arma- dillos, of Dipnoi and Echinodermis it is instructive to notice in the volumes of the Journal of Experimental Zoology how many embryological investigations sound the experimental note. There has been great activity not only as regards what is often called
