D.— ZOOLOGY. 93 



is the functioning of dogs and rabbits, with items from the frog. We know 

 a little about the heart of a dogfish, and about its haemoglobin, but 

 nothing of its respiration or the activities of its nervous system. 



Amongst the Mollusca we know a good deal about the food-collecting 

 mechanism and digestive enzymes of Lamellibranchs, and even in some 

 cases some details of the activities of the heart and the nature of their 

 respiratory pigment. But in no single case do we know even the outlines 

 of the whole physiology. 



We do not know how much food is eaten or the relative proportions of 

 proteins, fats, &c. We do not know how this food is utilised, how much 

 to maintenance, how much to growth, and so on. We have no real know- 

 ledge of the function of excretion, we do not know the blood volume, nor 

 the output of the heart under any circumstances whatsoever. We do not 

 even know the oxygen-carrying power of the blood as a whole, nor the 

 total consumption of oxygen and respiratory quotient in any one form. 



Until these things are known, in at least a few individual species of 

 each phylum, we shall not be in a position to understand the possibilities 

 of adaptation which each fundamental type of morphology holds out and 

 the real significance of the fitting of an animal to its environment. 



The reason why such a series of investigations has not yet been carried 

 out is clear ; to do so implies a long-continued and perhaps tedious 

 research invohang the modification of many different physiological and 

 biochemical techniques to enable them to be applied to new material ; 

 without holding out the bait of a promise of spectacular results. Far too 

 much work in comparative physiology has been no more than the partial 

 exploitation of a ' nice preparation ' found perhaps by a casual observation. 



But the ecological relationships of animals to their environments 

 present many aspects which are now capable of investigation by simple 

 physiological experiment. It would be a matter of extreme interest to 

 know something about the amount of water required by two mammals, 

 if possible members of different geographical races of the same species, 

 or at any rate neighbouring species, one from an arid, the other from a 

 more humid environment. To be valuable, such an experiment would 

 have to be carried out under carefully controlled conditions of humidity 

 and of temperature, and would necessarily involve an investigation of 

 the variations in the composition of the urine under different conditions. 

 Indeed this and all similar experiments would have to take into account 

 that power of adjusting their activities to circumstances which all animals 

 possess. 



But water requirements, and their variation imder different conditions 

 of humidity, important though they probably are, are only one of the many 

 things of which the effects of variations of mean temperature and range of 

 temperature, proportion of the year in which the temperature falls 

 below some point or exceeds some other, exposure to light, the chemical 

 nature of the food supplies, the possible absence or insufficient amount 

 of individual elements like phosphorus or iodine, are others which are 

 obviously open to relatively simple experiment. 



Only when such researches have been carried on for a number of pairs 

 of animals shall we have any real understanding of the significance 

 of the differences which separate one geographical race from another. 



