﻿BOYSEX RESERVOIR VERTEBRATE FOSSILS WHITE 199 



growth, during tlie period of postnatal development (ibid., p. 45). 

 That the thyroid (Goldzieher, 1939, p. 83) plays an important role in 

 influencing the morphogenic processes, particularly in the ossification 

 of the skeleton and in the growth of the teeth, has been demonstrated 

 by the administration of thyroid extract to hypothyroid children and 

 by thyroidectomy of normal laboratory animals. Its action is by no 

 means independent but is closely integrated with that of the pituitary, 

 parathyroid, and adrenals. Nor do these glands function only in com- 

 bination with each other but in combination with the other glands of 

 the system to maintain an endocrine balance and a favorable "internal 

 environment" or homeostasis (ibid., p. 11). 



In order to maintain homeostasis the endocrine glands must respond 

 to external factors. The changes in external environment that are 

 accompanied by changes in the activity of the glands are: Altitude, 

 temperature, climate, quantity and tyi)e of food, and accessory food- 

 stuffs such as mineral salts and vitamins. The data on the responses 

 to these factors are limited almost entirely to the clinical observations 

 on man and laboratory animals. These data indicate that when less 

 than radical changes in the external factors prove deleterious there 

 is a strong probability that an endocrine imbalance already existed 

 (Goldzieher, 1939, pp. 11-13). 



Of equal importance to the activities of the endocrine glands are 

 (he responses of the receptor tissues to the stimuli of the hormones. 

 These responses may be affected by a variety of factors, such as : The 

 condition of the tissues, ennervation, chemicals, and the age and stage 

 of development of the individual. With regard to the two last- 

 mentioned factors, which are probaby the most fundamental, little 

 is known except that the responses of the tissues are characteristic 

 of the stage of development. Thus, in a young and growing individ- 

 ual, the response to hormones is growth and maturation, while in the 

 adult the hormones are capable only of maintaining the orderly 

 function of the tissues (Goldzieher, 1939, p. 6). It is a well-known 

 fact that in animals there is a noticeable slowing down of growth soon 

 after puberty, though the postpubertal growth period may be as long 

 as the prepubertal. However, this may be due to the interaction of 

 the endocrine glands. 



In regard to the environment offered by this region during Lower 

 Eocene time. Van Houten (1945, pp. 442-444) characterizes it as a 

 humid lowland with a warm-temperate to subtropical climate sup- 

 porting a luxuriant vegetation of both woodland and savannah types. 



As to the soils, the j)arent rocks from which they were derived were 

 igneous (extrusives and intrusives) and sedimentary (limestones, 

 dolomites, shales, and sandstones) . Weathering processes would make 

 available phosphorus, potassium, calcium, and some sodium from 



