92 THE PROBLEMS OF EVOLUTION 



results of X-ray treatment of Drosophila also bear 

 witness to the possibility of external environment 

 reaching the chromosomes, whether somatic or 

 germinal. Nevertheless the great bulk of genetic 

 evidence demonstrates that the chromosomes are 

 usually able to maintain themselves within re- 

 markably narrow limits of variation, not to say 

 in an invariable state. This seems the goal of 

 organic activity. Change is a normal occurrence 

 under existing conditions, but wherever an ap- 

 proximately constant environment prevails we see 

 the tendency of the organism to be constant, and 

 everywhere in the organic world there is evidence 

 of the tendency toward preservation of that which 

 exists. Constancy in spite of conditions is a goal 

 but constant adjustment to conditions is a univer- 

 sal necessity. 



It is apparent at once that this is the major 

 problem of evolution, and that chromosomal func- 

 tions are a vital part of it. If we could know ex- 

 actly how their relations with the cytoplasm are 

 consummated, the problem would be simplified, 

 but that is obviously impossible at present. We 

 must be satisfied with the conclusions already 

 recorded as we proceed to the consideration of less 

 fundamental aspects of the relationship. 



Weismann's interpretation of the continuity of 

 the germ plasm implied isolation from the influ- 

 ences of the body. This idea has colored biological 



