404 AiSTNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



direct association between the parent and its offspring ceases at a 

 comparatively early stage of the development of the latter. This 

 does not mean that the parent ceases to have any influence over the 

 environment of its offspring as soon as the direct association between 

 the two ceases. One need only refer to the elaborate precautions 

 taken by birds to preserve a suitable environment about their young 

 after hatching. 



The direct association between parent and offspring persists in 

 viviparous animals until a much later stage in the development of 

 the latter. In these organisms, as in the former class, the environ- 

 ment of the young during the period of gestation is furnished by the 

 circulating fluid of the body of its parent. The variations of this 

 fluid are kept within certain limits by the regulatory mechanisms 

 of the parent. The young organism is thus provided with a medium 

 of constant properties while its own regulatory mechanisms are de- 

 veloping. In some viviparous animals the association between par- 

 ent and offspring ceases almost completely at birth, and the young 

 organism, having been provided with its own adjusting mecha- 

 nisms, as efficient as those of its parent, is left to adapt itself to its 

 new surroundings. 



Like most of their adaptive mechanisms, the devices of mammals 

 for the care of their young are more complex and effective than those 

 of other forms of life. In addition to the protective measures to 

 which allusion has been made, the mammals provide for their off- 

 spring a special food, milk, during part of their extra-uterine life. 

 The period for which this provision is made varies widely among 

 different species. In man it extends, under natural conditions, over 

 the greater part of a year, in some races much longer. 



This mechanism for the adaptation of one factor of the environ- 

 ment to the needs of the organism, represents one of the last stages 

 in adaptation by modifications of bodily structure and function. It 

 is interesting to observe how closely, in this latest adaptive mecha- 

 nism, certain properties of the environment are adjusted to the needs 

 of the organism. 



It should be remarked at the outset, however, that one of the most 

 striking properties of milk has probably no significance in relation 

 to its use as a food by the young organism. This is the fact that the 

 osmotic pressure of milk has a value very close to that of the body 

 fluids of the animal by which it is consumed. Milk probably owes 

 this property to the manner of its secretion from the body fluids 

 of the maternal organism. Before the milk is absorbed by the young 

 animal, certain of its constituents must undergo a process of diges- 

 tion or hydrolysis. The sum of the osmotic pressures of the prod- 

 ucts of digestion is considerably greater than that of the original 



