MENDEL’S LAW 303 
this other complementary allelomorph is introduced, 
however, by crossing with an individual which contains 
it, the feature previously hidden becomes visible, giving 
rise to the phenomenon which has long been familiar 
under the name of reversion on crossing. The demon- 
stration of these invisible factors, and of the fact that 
they also obey Mendel’s law with perfect regularity, 
is surely one of the most remarkable discoveries which 
have ever been made in the whole history of biology. 
This, again, is a piece of knowledge which may be of 
the very greatest importance, not only to breeders of 
bright flowers, some of which are already known to 
exhibit the phenomenon described, but also in all 
classes of breeding work where similar facts doubtless 
await discovery. ; 
To the man of science, however, the practical aspect 
of these achievements will be of little account in com- 
parison with the importance of their application to the 
advance of human knowledge in that most fascinating 
of scientific studies—biology. Let us, then, turn to 
consider the way in which Mendel’s discovery affects 
other branches of biological science. 
We have so recently had occasion to point to the 
remarkable coalition between Mendelism and cytology 
that little more need be said here upon the subject. 
Mendel’s theory has, indeed, thrown a flood of light 
upon the meaning of the microscopic phenomena 
exhibited by the minute constituent parts of the cells 
of living organisms, phenomena the meaning of which 
could only be vaguely guessed at previously to the 
introduction of the new method. 
