CHARACTERISTICS: RELATION TO ENVIRONMENT 24 1 



Of greatest Interest from our present point of view is 

 the fact that external conditions may Induce this same trans- 

 formation. If the axolotl Is driven to come out on land 

 under certain conditions of temperature and the like, it 

 transforms into an Amblystoma, just as It does when fed 

 thyroid. There Is little doubt that what these conditions do 

 is to cause the thyroid of the animal to discharge its secre- 

 tion into the blood, and that this Induces the transforma- 

 tion. Thus the axolotl may have either one of two very 

 diverse sets of characteristics, depending on the conditions 

 that it meets during development. 



Characteristics Affected both by Genes and by 



Environment 



Any of the characteristics of the organism may be al- 

 tered by changing its genes: this we have seen in earlier 

 chapters. Many of the same features that can be altered by 

 changing the genes can likewise be altered by appropriate 

 changes in the environment. Characteristics do not fall 

 into two classes, one exclusively hereditary (or dependent 

 on genes), the other exclusively environmental; but any 

 characteristic is affected both by the materials of which the 

 organism is composed, and by the action of the conditions 

 on these materials. 



Yet, as we shall see, in practice some characteristics are 

 more readily altered by environmental conditions than are 

 others. Certain characteristics owe most of their peculiari- 

 ties to diversities among genes. Others are readily affected 

 both by genes and by environment. Still others depend 

 mainly on environmental conditions. 



Particularly Illuminating are the cases in which charac- 

 teristics depend in marked degree both on genes and on 

 environmental conditions, so that they are altered by 

 changes in either. A number of well known cases of this 



