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 
