104 The Unity of the Organism 



down as a law. As thorough a chemico-zoological investi- 

 gation of milk as that made by Reichert and Brown of 

 blood, ought to yield highly interesting results, for not only 

 connnon knowledge, but technical knowledge as well, obtained 

 in connection with the dairy industry recognizes that even 

 as between different breeds of cows the milk differs in con- 

 stitution. Jersey cows for example, produce milk contain- 

 ing a larger proportion of butter fat than do Ayrshires, and 

 some at least of these breed-differences in milk cannot be 

 explained on the basis of differences in food or other en- 

 vironmental factors, powerfully as these do undoubtedly 

 influence milk. 



Attention may be called to the extreme chemical sensitive- 

 ness of this fluid as a registering instrument. "Circum- 

 stances tending to cause discomfort usually lower the pro- 

 portion of volatile acids present in the butter-fat, but the 

 variation in the composition is very irregular, and appears 

 to depend partly upon the nervous temperament of the 

 cow." ^^ And there is ample evidence that the character of 

 the milk of women may be so changed by nei-vous and mental 

 conditions as to become unfit for the nursing babe.^^ 



{e) Comparative Chemistry of Digestive Enzymes 



Another great field of chemico-zoological research has 

 recently been opened up by studies on the enzymes of diges- 

 tion. The investigations of this sort which we will notice 

 have been specially prosecuted by the Swedish chemist, S. 

 Hedin. The results are given in outline by A. Hardens 

 and from this the following statement is, in the main, 

 drawn.-*' The problem concerns rennet, the familiar milk 

 clotting substance produced in the calf's stomach, and 

 Hedin's results are not influenced so far as I can see, by 

 the much debated question of whether or not pepsin and 

 rennin are tw^o entirely distinct bodies. It is shown that the 



