No. 543] 



PROBLEMS OF EVOLUTION 



]->: 



On the other hand, selection of favored races and elimi- 

 nation of the unfit is still the only natural explanation of 

 fitness, of adaptation, in organisms. As a species-form- 

 ing factor selection is probably of less importance than 

 Darwin supposed ; as a possible explanation of the won- 

 derful adaptations which all living things exhibit it seems 

 to be all important; but extensive experimental investi- 

 gations of the causes of adaptation are greatly needed. 



IV 



The microscopic study of the germ cells during the past 

 twenty-five years— their growth, maturation, union in 

 fertilization, and their subsequent development— lias fur- 

 nished material of the greatest importance for the com- 

 prehension of the mechanism of heredity and evolution, 

 and yet almost everything in this field remains to be done. 

 The parts played by the different constituents of the cell 

 in assimilation, regulation and heredity are still in doubt, 

 and in spite of many alluring hypotheses we know prac- 

 tically nothing about the way in which hereditary char- 

 acteristics arise from the germ. The study of the cellu- 

 lar basis of heredity has to a large extent been guided 

 and influenced by our knowledge of the gross phenomena 

 of heredity, and this must always be the case; but the 

 brilliant discoveries of the last few years as to the cellu- 

 lar basis of sex show the great assistance which the study 

 of cytology may render to the science of genetics. Many 

 interesting experiments have been made upon the germ 

 cells in the attempt to shift dominance, to modify inher- 

 itance, to create new characters ; in a few instances it has 

 been shown that certain modifications of the embryo or 

 adult organism follow certain modifications of the germ, 

 but in no instance has it been shown that such modifica- 

 tions are inherited and are consequently of evolutionary 

 value. 



Not merely the constitution of the germ and the ways 

 in which this mav be modified, but also the precise man- 

 ner in which the structures of the germ become trans- 



