172 MILK AND ITS HYGIENIC RELATIONS 



find the milk of two species which is sufficiently similar in com- 

 position for these differences not to require consideration. 



These differences in composition have led many people to believe 

 that goats' milk, which is nearer in composition to that of human 

 milk, should, theoretically speaking, be preferable for infants to 

 cows' milk. 



Condition of Young Animals at Birth. The condition of young 

 animals when born into the world varies within very wide limits. 

 Some are born comparatively mature, while in others the striking 

 feature is the immaturity of condition at birth. Guinea-pigs, for 

 instance, are active almost at once after birth. During the first 

 few hours they are found to be capable of eating almost anything 

 that they can find in their cage. They receive mother's milk for 

 a short time only, and at no period are they entirely dependent 

 upon their mother. Throughout their existence, from birth on- 

 wards, they take vegetable substances as well as mother's milk. 

 On the other hand, such animals as kittens and rabbits are born 

 in a condition in which they are entirely dependent upon their 

 mother for care and for sustenance in the way of milk. Evidently 

 the needs of organisms in such varied conditions of maturity must 

 be different. 



It is to some extent irrelevant to compare, for instance, the 

 nutritive value of raw and boiled cows' milk upon guinea-pigs with 

 a view to ascertaining the value of this food for infants. 



Rate of Growth. The rate of growth of most young animals is 

 much more rapid than that of the human infant. Adult life is 

 reached more rapidly. Animals such as rats and mice are mature 

 in a very few weeks from the time of their birth, and are capable of 

 bringing families into the world within two or three months of 

 their own birth. Compared with that of the human infant, who 

 is dependent for its existence upon milk for a considerable number 

 of months after birth, the development of these animals offers no 

 parallel conditions. 



Additional Experimental Difficulties. Several additional experi- 

 mental difficulties arise. It is well known that where only one 

 or two animals can be secured for experiment, the margin of 

 error is extremely great. Should one of them die, the experiment 

 is practically valueless. It is not easy, however, to secure a large 

 number of animals of exactly the same age. Hence many observers 

 have been obliged either to work with an unduly small number of 

 animals, or they have found it necessary to utilise animals whose 

 ages, although roughly similar, were not identical. In animals 

 whose dependence upon the mother for nutriment is reckoned 

 only in days, evidently a few days or a week's difference of age 

 may destroy the value of the experiment. 



These remarks show that too much reliance must not be placed 

 upon animal experimentation when it is desired to obtain evidence 

 relating to the nutritive value of food for infants. At the same 



