286 GENETICS IN RELATION TO AGRICULTURE 
their germ cells are similar. Hence they breed true in the main but 
occasionally throw the new combinations of diverse elements which have 
come to be known as ‘‘mutants.”’ 
In conclusion, it may be well to state our reasons for restricting the 
term, mutation, to those changes in specific factors, which result in the 
appearance of new Mendelizing characters. This term was used by de 
Vries to designate a more or less comprehensive change which appeared 
suddenly, without warning, giving the impression that a full-fledged new 
species had sprung from a pure, constant, old species much as Athena 
sprang from the head of Zeus. We cannot conceive of new species 
originating in this way except in certain exceedingly rare cases, which 
fall under the two categories already described and illustrated, viz., 
(1) single factor mutations having such a profound manifold effect that 
the new form would be generally recognized as a distinct species, and (2) 
chromosome aberrations during mitosis or meiosis. We have found that 
the majority of the new forms derived from (nothera lamarckiana do 
not fall into either of these categories and that the most reasonable 
explanation of their origin is based on the assumption that @. lamarck- 
zana is of hybrid origin. Therefore, if the term, mutation, is to retain 
the meaning originally given it by de Vries, we cannot continue to classify 
the majority of new Cénotheras or other organisms resulting from 
hybridization as mutations. 
On the other hand, the fact that most discontinuous, inheritable varia- 
tions are caused by alterations in genetic factors and that these factor 
mutations play an important réle as one means for organic evolution, 
seems to justify their recognition as mutations in the strict sense. By 
limiting the meaning of mutation as we propose all the objectionable im- 
plications previously connoted by the term are removed. The desir- 
ability of accomplishing this has been indicated by Agar, who states: 
“The greatest opposition to modern views of genetics has come from those 
who consider that they have taken away the philosophical basis of the theory 
of evolution and especially of the evolution of adaptation. For, while mutation 
could quickly bring about specific diversity, the evolution of complex adaptive 
structures is undoubtedly most easily grasped when the inheritable variations 
presented to natural selection are minute and abundant. This difficulty, though 
real, would undoubtedly have assumed smaller proportions had it not been for 
the natural fact that the earliest mutations studied were large morphological 
ones, and consequently that these have become fixed in many minds as types of 
mutational change.” 
There is now abundant evidence that genetic diversity is expressed 
in minute morphological and physiological differences, and hence that 
mutations produce those small inheritable differences logically required 
for the explanation of adaptation through natural selection. 
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