MENDEL'S LAW 269 



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. 



