The extreme application of the
principle would no doubt disturb the limits between many species and
varieties as now recognized. It is not to be forgotten however that all
taxonomic distinctions, which have not been confirmed by physiologic
tests are only provisional, a view acknowledged by the best
systematists. Of course the description of newly discovered forms can
not await the results of physiologic inquiries; but it is absolutely
impossible to reach definite conclusions on purely morphologic evidence.
This is well illustrated by the numerous discords of opinion of
different authors on the systematic worth of many forms.
Assuming the above mentioned principle as established, and disregarding
doubtful cases as indicated, the term progressive evolution is used to
designate the method in which elementary species must have originated.
It is the [249] manner in which all advance in the animal and vegetable
kingdoms must have taken place, continuously adding new characters to
the already existing number. Contrasted with this method of growing
differentiation, are the retrogressive modifications, which simply
retrace a step, and the degressive changes in which a backward step is
retraced and old characters revived. No doubt both of these methods have
been operative on a large scale, but they are evidently not in the line
of general advancement.
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