220 MENDELISM 
formation of the gametes in the offspring its members 
would separate as perfectly definite entities, to re- 
combine when these gametes meet once more with their 
corresponding mates. Such a definite segregation of 
characters taking place within a pure strain would be 
very difficult of absolute demonstration, but it is hard 
to avoid the conclusion that this is a true deduction 
from the facts observed when cross-breeding takes 
place. Such a segregation would formerly have been 
thought a very small assumption in comparison with 
that of the segregation of pairs of allelomorphs of 
which no trace is externally visible, and yet the latter 
assumption has now been shown to be perfectly well 
established. 
This idea of unit characters, capable of being inherited 
independently of one another, is one of the most 
important conceptions which has ever been introduced 
into the science of biology, and the introduction of it 
has followed as the direct result of Mendel’s work. It 
is a conception which has led to a complete change in 
our ideas of heredity, since we no longer look upon 
the individual as a unit, but find ourselves compelled 
to study separately the independent characters of 
which the individual is built up. The idea of the 
individual as a living mosaic—an idea put forward long 
ago by Naudin with only a partial realization of its 
significance—has thus returned to us. In this con- 
nection a curious problem presents itself. What 
would be left if we could imagine all the separable 
characters of a living creature as having been taken 
away? Would there, or would there not, be any 
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