THE TREND OF EVOLUTION 161 



balance, or integration, and the adaptability to external condi- 

 tions. Furthermore, in the higher types of organization the 

 mutual dependence of parts becomes so great that when one 

 of these is injured or breaks down it carries with it to destruc- 

 tion the entire organism. As Professor Minot used to say, 

 "Death is the price we pay for our differentiation." Germ 

 cells and embryonic cells are potentially immortal, but tissue 

 cells are not self-sustaining and the more highly they are dif- 

 ferentiated the greater is their dependence and the more 

 certain is their ultimate death. 



The record of the rocks is full of instances in which groups 

 of organisms have progressed to greater and greater com- 

 plexity and have then become extinct. The roads of progress 

 are strewn with the remains of creatures fearfully and won- 

 derfully specialized, but which were unable to preserve inter- 

 nal balance or to adapt themselves to new environments and 

 which therefore perished. Indeed, in practically every instance 

 the road of ever increasing progress ends in extinction. But 

 just as in the succession of generations highly differentiated 

 cells and individuals die and new generations arise from rela- 

 tively undifferentiated eggs, so when highly differentiated 

 species become extinct new lines of progress start from gen- 

 eralized rather than from highly specialized types. 



It is of course conceivable that differentiation might go on 

 indefinitely in any line, the elephant might get a longer trunk, 

 the giraffe a longer neck, and man a larger and larger brain, 

 but while such things are conceivable they are not practicable 

 for the reasons named. In any line of evolution progress is 

 most rapid at first and then it gradually slows down until it 

 stops, and in every well-tried path of evolution, progress has 

 practically come to an end; further progress, if it occurs, must 

 be in new lines and from relatively undifferentiated stock. 



Progressive Evolution of Man. There have been three 

 main lines of human evolution, physical, intellectual, social, 



