THE RATE OF GROWTH 87 



be. And indeed, if we take any such partial measure, 

 how could we compare different forms with one an- 

 other? The height of a horse is not comparable to 

 that of a man ; the height of a caterpillar is not 

 comparable to that of any vertebrate. Naturally, 

 therefore, we take to measuring the weight, which 

 represents the total mass of the living body, and en- 

 ables us at least with some degree of accuracy to 

 compare animals of different sorts with one another. 

 Now in studying this question of the increase of 

 weight in animals, as their age increases, it is obvi- 

 ously desirable to eliminate from our experiments all 

 disturbing factors which might affect the rate of 

 growth or cause it to assume irregularities which are 

 riot inherent either in the organisation of the animal 

 or in the changes age produces. The animals which 

 belong to the vertebrate sub-kingdom, of which we 

 ourselves are members, can be grouped in two large 

 divisions according to the natural temperature of 

 their bodies. The lower vertebrates, the fishes, 

 frogs, and their kin, are animals which depend for 

 their body temperature more or less on the medium 

 in which they live. The other division of vertebrate 

 animals, which includes all the higher forms, are so 

 organised that they have within certain limits the 

 power of regulating their own body temperature. 

 Now it is easily to be observed and any one who 

 has made observations upon the growth of animals 

 can confirm this that animals otherwise alike will 

 grow at different speeds at different temperatures. 



