EVOLUTION TODAY 437 



Environmental change may also be due to the shifting popula- 

 tion, as mentioned in Chapter XIII, but whatever its cause, organ- 

 isms in contact with it must meet the changed conditions or die. 



Duration of Environment. When biologists have experimented 

 with environmental effects on organisms in the past they have 

 had to be contented with a study covering relatively few 

 generations and the characters available for study have been 

 relatively unimportant. In long phylogenetic series we find that 

 the gradual development of a species is correlated with a lasting 

 chmate, or with a climate which is gradually changing in a way 

 favorable to the existence of the changing organism. The evolution 

 of species has not occurred in a few years nor under frequently 

 changing conditions. 



Whether incidental changes or direct adaptive responses in 

 organisms have been the initial step in the evolution of species we 

 cannot say. This is the essence of the conflict between the views 

 of Darwin and Lamarck. If changes have appeared in organisms, 

 giving rise to an assemblage of variations, changing environment 

 may emphasize their relative values in such a way as to cause 

 natural selection. On the other hand it is obvious that some 

 changes come about as direct adaptive response to a new condition. 

 Organisms have unlimited opportunity for contact with new 

 environmental stimuli through migration, climatic change, and 

 fluctuations in the organic environment, hence there is always a 

 chance for the occurrence of new characters of both kinds. 



Are Any Characters Permanent? Since all changes in the 

 organism are the product of hereditary genes responding to 

 stimuli from some part of the complex environment, the question 

 arises, how do they differ in permanence? If characters appear 

 in response to conditions of the internal environment, then o})vi- 

 ously their reappearance in each generation will be reasonably 

 certain. However even the internal environment is not permanent ; 

 it is subject to modification both from within and from without. 

 If the proper condition in the body fails to develop, even appar- 

 ently hereditary characters fail to appear. A cretin may have a 

 normal mental heritage, ])ut the genes responsible for mental 

 development do not find the proper conditions in its body for 

 their expression. The deficiency may be corrected by the timely 

 administration of thyroid extract and the heritage then responds 

 normally. 



