256 Experimental Zoology 
Man is taken as the standard, and the time required by the other 
forms to double their birth rate is given in fractions of that of 
man. 
pauls ae Fat SUGAR ALBUMEN CaO . P.O5 
Man I 3.5 6.6 1.9 I I 
Horse $ I.I 6.1 2.3 4 3 
Ox q 4.5 45 4.0 — 5 4 
Pig io 6.9 2.0 6.9 - - 
Sheep re 10.4 4.2 7.0 8 9 
Dog aE 10.6 3.1 8.3 I4 Io 
Cat v5 3:3 4.9 9.5 = = 
It will be seen that the rate of growth is proportional to the 
amount of albumen in the milk. Thus the kitten doubles its 
weight in 1/33 of the time required by the human infant, and the 
milk contains five times as much albumen. It is improbable 
that the results are due solely to this factor, but that an internal 
factor largely regulates the growth of the young. Were this not 
the case, we should find that a baby fed on sheep’s milk would 
doubt its weight eighteen times as fast as a baby fed on human 
milk. 
Stimulants affecting Growth 
I have already stated that the rate of growth is not simply a 
question of the amount or of the kind of food. In fact, growth 
depends in some cases on a response to a stimulus, and the amount 
of food stuffs converted into tissues sometimes depends on the 
presence of a stimulus. For example, the embryo in the uterus 
of the mammal appears to supply such a stimulus. Many 
chemical substances ‘‘that are not themselves food may stimulate 
the growth processes.’’ Just as certain poisons accelerate the 
movements of animals, so may they also accelerate metabolic 
processes and lead to increased growth. ‘Schultz found that 
various poisons, such as corrosive sublimate, iodine, bromine, 
arsenious acid, increase the activities of yeast in fermentation.’ 
* Davenport, “ Experimental Morphology.” 
