SUMMARY 225 
one pair of allelomorphs has taken part in the cross, the 
members of each pair are found, as a rule, to undergo 
this process of segregation quite independently of all 
the other pairs. 
The result of this phenomenon of segregation is 
that we find our conception of what constitutes purity 
in a strain of animals or plants to be completely 
altered. We now know that purity does not depend 
upon the length of time during which the race has 
exhibited a constant character. A strain of absolute 
purity may arise from the second generation of a cross. 
Such a pure strain may show an entirely new com- 
bination of the parental characters. But this is so 
far the only kind of novelty which we can produce at 
will. We know almost nothing as to the method by 
which entirely new characters arise. We can only 
take advantage of such characters when they happen 
to make their appearance. 
I would draw special attention to the definiteness of 
the characters with which we deal. We do not evoke 
improved features by gradual selection ; the characters 
are either there or they are not. Let it be further 
remembered that every process of this kind which has 
been worked out in the case of a plant can be paralleled 
by similar phenomena taking place in some one or 
other of the higher animals. 
On the mind of a biologist familiar with what was 
known of heredity only ten years since these facts 
must fall with a sense of complete novelty. The 
ideas current even so short a time ago are not so much 
extended, or even altered, as replaced by an entirely 
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