10 8 AGE, GROWTH, AND DEATH 



minoid, or proteid material contained in the milk. 

 You will observe that for man the proportion is 

 lowest, 1.6 per hundred parts; the horse has a little 

 more 2 ; cattle 3.5 ; and so the values run. In 

 other words, it is obvious that the less the proteid in the 

 milk, the longer does the species require to double its 

 weight. This looks at first sight as if there were a 

 relation between the composition of the milk and the 

 period of growth of the animal ; but you know very 

 well that if you take the milk of a cow, which is very 

 much richer in proteid material, and feed it to a baby, 

 a human baby, that baby does not grow at the same 

 rate as the young cow, but grows at the human rate. 

 It is obvious, therefore, that it is somewhat more 

 complicated than a mere question of food supply. 

 We have here in fact one of the beautiful illustrations 

 of the teleological mechanism of the body. These va- 

 rious species have their characteristic rates of growth, 

 and by an exquisite adaptation, the composition of 

 the mother's milk has become such that it supplies 

 the young of the species each with the proper quantum 

 of proteid material which is needed for the rate of 

 growth that the young offspring is capable of. It is 

 a beautiful adjustment, but there is not a causal rela- 

 tion between the proteid matter of the mother's milk 

 and the rate of growth of the young. It is an example 

 of correlation, not of causation. 



We pass now to the next of our slides (Fig. 37), 

 which carries us to the study of our own species. It 

 is not possible at the present time to represent in any 



