4 INTRODUCTION 



of the most fundamental parts of physical science, as 

 I think we may fairly call the branches of electricity 

 and molecular physics, seem at present to be under- 

 going modifications which bid fair to bring about a 

 complete revolution in current ideas upon these sub- 

 jects. It is highly probable that these results will 

 ultimately lead to a considerable modification in pre- 

 vailing notions about living things ; but the new 

 developments have yet to reach biology through the 

 channels of organic chemistry, physiology, cytology, 

 and the like, and at present we do not know what the 

 result of this influx is likely to be. These considera- 

 tions need not, however, detain us, for the new know- 

 ledge of variation and inheritance, of which it is pro- 

 posed to give some account, is largely concerned with 

 the grosser characters of organisms, so that ultraminute 

 structures may be left alone for the present until the 

 stream of physical knowledge stirs them into greater 

 prominence. So much is this the case with the study 

 of variation and inheritance by experimental methods 

 at the present day, that this science is treated by some 

 with a fine contempt, because its tools are those of the 

 breeder and gardener, and because the assistance of 

 the compound microscope may often be laid aside for 

 days together. Yet this applies only to one aspect of 

 the subject, and the microscopic study of the embryonic 

 rudiments of organisms, going hand in hand with the 

 experimental observation of adult structures, is rapidly 

 leading to a clearer understanding of the processes of 

 heredity. 



The problem which those who are engaged in this 



