226 MENDELISM 
new set of ideas. And it may be remarked in passing 
that the biologist of fifty years ago and more was much 
nearer to our present line of inquiry. 
We have seen enough to enable us to recognise 
very clearly the vital importance of an understanding 
of the constitution of the gametes in all questions of 
heredity. There must exist in the gametes, in an 
uncombined condition, those units which by their 
combination in zygotic organisms lead to the appear- 
ance of the characters which we can recognise. But 
we have seen that, owing to the appearance of domi- 
nance and other kindred phenomena, the visible external 
characters of an organism are not a complete guide to 
the nature of its gametes. It is only by careful 
breeding that we can distinguish the heterozygote from 
the pure dominant form—to take the simplest possible 
example of this difficulty. For this reason it has now 
become the chief business of the student of heredity 
to determine by experiment what combinations of 
allelomorphs are present in the gametes of the indi- 
viduals with which he is working. 
The behaviour of these allelomorphs has now been 
disentangled in many cases of very considerable com- 
plexity ; and all such cases as have been so far examined 
in detail have proved explicable in terms of a larger or 
smaller number of allelomorphic pairs, all of which 
obey Mendel’s law—with the single exception of those 
cases in which coupling between the allelomorphs of 
different pairs introduces a slight further complication. 
Although it is perhaps scarcely probable that Mendel’s 
law will ultimately prove universal in its application, 
