EVOLUTION BY HYBRIDIZATION 
EDWARD C. JEFFREY 
Harvard University 
Not long ago we were told that the investigation of the problems 
of evolution had left behind the inexact if broad phase of study in the 
field and had now entered upon the more accurate and satisfactory 
stage of quantitative elaboration under laboratory conditions. Leay- 
ing aside the question whether whatever exactitude in connection 
with this tendency has not been more than offset by a corresponding 
narrowness of outlook, it is now quite apposite to inquire if the experi- 
mental methods of the physiologist are in reality in the position to 
supply final light upon the fundamental problems of evolution. The 
judging of living beings by what they do rather than by what they 
are, has made notable progress in recent years. We are often told for 
example that an organ is the tool of a function and consequently 
should be defined by its performance rather than by its organization. 
I need not point out the essential fallacy of the physiological definition 
of an organ cited above. It obviously breaks down the moment it is 
used on any wide range of facts. 
Perhaps the most striking illustration of depending overmuch 
upon physiological data is supplied in connection with present investi- 
gations upon the all important question of the origin of species. It 
is practically universally assumed in genetical studies, that the capacity 
to breed true under exacting experimental conditions is the most 
reliable criterion of good species. It has for example assumed that 
breeding results obtained with Oenothera and Drosophila are of funda- 
mental importance for the science of biology. By those of us who 
have neither been intoxicated with the cult of the evening primrose 
nor bowed the knee in the temple of the god of flies, this conclusion 
will in general be held undemonstrated. We must obviously know a 
good deal more about the antecedents of those forms which have 
been raised in recent years to the dignity of veritable biological touch- 
stones, before we can admit the validity of the far-reaching con- 
clusions drawn from their genetical behavior. 
The question of the origin of the species is as much with us ae 
as it was at the time of the publication of Darwin’s epoch-making 
work. Darwin himself ultimately ventured no explanation of the 
causes of the changes concerned in the formation of new species, but 
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