64 THE EVOLUTION OF MAN 



increases and this change in the relative size of nucleus and 

 cytoplasm constitutes the chief structural difference between 

 the cells of the young and of the old. A decreasing rate of 

 growth is also accompanied by a diminution in the water 

 content of the body, as is shown by the fact that in the first 

 month of prenatal life the percentage of water in the body 

 is 97.5 per cent, at birth it has become 74.7 per cent, and at 

 maturity 58.5 per cent. 



While man and the higher vertebrates have a definite period 

 of growth some fishes and amphibians continue to grow 

 throughout life. The forces which stimulate or inhibit growth 

 are largely unknown. Evidence is accumulating to show that 

 certain of the internal secretions, notably those given off by 

 the thyroid and pituitary glands, play a large part in the con- 

 trol of growth. It is believed that too much secretion of the 

 latter may increase the growth of local areas of the body such 

 as the hands and face. This condition is to be seen in the rare 

 disease known as acromegaly. On the other hand, a dimin- 

 ished secretion of the thyroid results in a failure of growth of 

 the entire body as well as a lack of mental development, a 

 condition known as cretinism. Various factors such as un- 

 favorable climatic conditions, poor food, lack of proper pro- 

 teins and vitamines, and severe illness retard growth. 



However, these factors do not offer any explanation as to 

 why most animals stop growing after reaching the size of the 

 species. A mouse never grows to the size of an elephant, nor 

 does an elephant remain as small as a mouse. It is apparent 

 that there is a fundamental hereditary factor involved which in 

 some way, possibly through the internal secretions, controls 

 growth. That this hereditary influence has not always re- 

 mained constant is shown in the size variation of the same 

 species in different geologic ages. This is notably shown in 

 the evolution of the horse. The question of growth is closely 

 associated with the power of regeneration, that is, the ability 



