EVOLUTION TODAY 431 



Raise it to a high temperature and its substance coagulates — it 

 dies. But keep it at 39° C and dev(>lopnient begins. Temperature 

 is one of the common factors of environment, and here we see that 

 the correct temperature is just as essential to the initial develop- 

 ment of the chick as the heritage borne by the egg. 



In our own bodies we may see the unfolding of hereditary char- 

 acters without obvious dependence upon the outer world. We grow 

 from day to day throughout childhood. It is true that growth de- 

 pends to a great extent upon the food that we cat, but some persons 

 fail to grow in spite of sufficient food and others grow to gigantic 

 size. Our mental developm(>nt increases, but in some individuals 

 this is not the case, although they may have a perfectly sound men- 

 tal inheritance. The characters of sex appear. We can see in these 

 cases only an indication of the fact already mentioned, that the 

 body itself provides stimuli for the activation and control of its 

 various parts. Some characters develop in response to these 

 stimuli alone. 



The environment, therefore, is not merely that which is outside 

 of the individual. Any organ in the body is as truly a part of 

 the en\aromiient of other organs as physical factors or living 

 things are a part of the environment of a plant or animal. The 

 environment may properly be subdivided into an external envi- 

 ronment, made up of the physical and organic, and an internal 

 environment which is estal)lished by the individual's own body. 



The essential effects of these environments are the same. Each 

 provides certain stimuli to which organs or organisms may respond 

 according to their inherited capacity. Glands in our bodies, food 

 which we secure from our organic environment and sunlight from 

 the physical environment all contribute to our normal growth. 

 The unfortunate individuals called cretins are mentally and 

 physically retarded because of deficiency of the thyroid gland, })ut 

 stunted growth and imperfect development may also come from 

 malnutrition and the imperfect bone development called rickets 

 may be due to lack of sunlight or a diet deficient in vitamines. 



The Source of Organic Characters. The various parts and 

 functions which characterize a given species are the most definite 

 indication of its heritage, yet it is evident that they are a product 

 not merely of the heritage, but of the heritage responding to 

 environmental stimuli. 



In the discussion of acquired characters it has usually been 



