There are a great many cases however, in which the morphologic cause of
the dissimilarity is not so easily discerned. But there is no reason to
doubt that most of them will be found to conform to the rule on closer
investigation. Therefore we can consider the following as the principal
difference between elementary species and varieties; that the first
arise by the acquisition of entirely new characters, and the latter by
the loss of existing qualities or by the gain of such peculiarities as
may already be seen in other allied species.
If we suppose elementary species and varieties originated by sudden
leaps or mutations, then the elementary species have mutated in the line
of progression, some varieties have mutated in the line of
retrogression, while others have diverged from their parental types in a
line of depression, or in the way of repetition. This conception agrees
quite well with the current idea that in the building up of the
vegetable kingdom according to the theory of descent, it is species that
form the links of the chain from the lower forms to the more highly
organized later derivatives. Otherwise expressed, the system is built up
of species, and varieties are only local and lateral, but never of real
importance for the whole structure.
[142] Heretofore we have generally assumed, that varieties differ from
the parent-species in a single character only, or at least that only one
need be considered.
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