62 SCIENCE PROGRESS 



the circulation in fish (i). The relative heart-size is accordingly 

 greater in amphibians and reptiles. In the frog (/?. tcniporaria) 

 and in a crocodile the heart was found to be about 0*4 per 

 cent, of the body-weight and to be nearly o"8 per cent, in the 

 common snake (7). But we do not as yet know what the 

 tissues take most from the blood in these lower vertebrates ; 

 we only know for a certain number of species of fish and 

 amphibians and for the crocodile (6) that they take very little 

 oxygen and that the rate at which this at least is supplied is not 

 likely to cause difficulty. Neither has anything yet been ascer- 

 tained about differences in pulse-rate in different genera of 

 amphibians nor in those of any of the different classes of 

 reptiles ; so that we have not the material for deciding whether 

 the frequency with which the heart beats has become one of 

 the factors in natural selection. In the few species of am- 

 phibians and reptiles (6) for which — sporadically — the frequency 

 is known, it seems to be not very different from what it is 

 in fish — i.e. varying (and varying in individuals of the same 

 species) between about 20 and 80 per minute at ordinary room 

 temperature. 



In birds and mammals the case is different. We not only 

 know that the tissues take a great deal of oxygen from the blood, 

 but that those of small animals take much more than those 

 of large ones ; and we can assign a reason. Birds and mammals 

 are able to maintain a nearly constant temperature whatever 

 that of their surroundings may be. They are homocothennic or 

 (the temperature they maintain being usually higher than that 

 of the environment) " warm-blooded " animals ; they have 

 in consequence to generate more heat than those animals 

 which maintain no constant temperature — the poikolothermic or 

 " cold-blooded " animals — and to try to prevent loss of heat. 

 To generate heat the muscles — the chief heat-producing organs 

 of the body — require oxygen, and they take it from the blood 

 according to their need, the need being greatest in those species 

 or individuals in which the loss of heat is greatest. The maximum 

 loss is of course in those animals in which the surface exposed 

 to the environment is largest in proportion to the mass of the 

 animal — i.e. the smaller the animal the more heat must it give 

 off, other things being equal, to a colder environment, and 

 to maintain a constant body temperature the more heat must 

 it generate and the more oxygen must its muscles consume. 



