CONDITIONS FOR DEVELOPMENT 289 



when a suitable food supply — suitable in quantity as well as 

 in quality — is provided. Overfeeding, especially in animals, 

 will affect the development, as well as underfeeding, by affect- 

 ing the health of the digestive organs or of the excreting 

 organs, or by leading to a deposit of fat in one or more organs. 

 Certain special substances, we have already found, may have 

 a direct influence upon the development as well as upon the 

 growth and activities of the body. 



336. Chemical influences. The direct influence of chemical 

 substances upon the growth and development of organisms is 

 best observed in the lower plants and animals, in which there 

 is no great modification of the received material before this 

 reaches the protoplasm. Nevertheless we have already learned 

 that thyroid extract can influence the development of the body 

 of the frog (p. 285), and it is well known that a derangement 

 of the thyroid gland affects the development (especially of the 

 central nervous system) in human beings. 



Over forty years ago a Russian observer reported that two 

 species of a small crustacean of the genus Artemia could be 

 converted one into the other by a change in the amount of 

 salt in the water. The form that usually lived in the less salty 

 water acquired some of the characters of the other form on 

 being placed where evaporation increased the relative amount 

 of salt in the water. And the second form acquired some of 

 the characteristic structures of the first when placed in a brine 

 that was gradually diluted by the addition of fresh water. 

 Under natural conditions that sometimes bring about a gradual 

 dilution of sea water by rains, or a gradual concentration of 

 waters by evaporation, modifications in the development of 

 mollusks, arthropods, and other animals have been observed. 

 These modifications extend to forms, colors, shells, bristles, 

 and other parts. 



By changes in the chemical condition of the medium, 

 experimenters have made the eggs of certain fish develop into 

 animals having a single eye in the middle of the head ; and 



