EVIDENCES FROM CLASSIFICATION I2I 
importance. These resemblances, though so intimately connected 
with the whole life of the being, are ranked as merely “adaptive or 
analogical characters’’: but to the consideration of these resemblances 
we shall recur. It may even be given, as a general rule, that the less 
any part of the organisation is concerned with special habits, ‘the more 
important it becomes for classification. As an instance: Owen, in 
speaking of the dugong, says, “The generative organs, being those 
which are most remotely related to the habits and food of an animal, 
I have always regarded as affording very clear indications of its true 
affinities. We are least likely in the modifications of these organs to 
mistake a merely adaptive for an essential character.” With plants 
how remarkable it is that the organs of vegetation, on which their 
nutrition and life depend, are of little signification; whereas the 
organs of reproduction, with their product the seed and embryo, are 
of paramount importance! So again in formerly discussing certain 
morphological characters which are not functionally important, we 
have seen that they are often of the highest service in classification. 
This depends on their.constancy throughout many allied groups; and 
their constancy chiefly depends on any slight deviations not having 
been preserved and accumulated by natural selection, which acts only 
on serviceable characters. 
WHAT IS A SPECIES? 
‘“‘Each kind of animal or plant, that is, each set of forms which 
in the changes of the ages has diverged tangibly from its neighbors, 
is called a species. There is no absolute definition for the word 
species. The word kind represents it exactly in common language, 
and is just as susceptible to exact definition. The scientific idea of 
species does not differ materially from the popular notion. A kind of 
tree or bird or squirrel is a species. Those individuals which agree 
very closely in structure and function belong to the same species. 
There is no absolute test, other than the common judgment of men 
competent to decide. Naturalists recognize certain formal rules as 
assisting in such a decision. A series of fully ‘intergrading forms, 
however varied at the extremes, is usually regarded as forming a single 
species. There are certain recognized effects of climate, of climatic 
isolation, and of the isolation of domestication. These do not usually 
make it necessary to regard as distinct species the extreme forms of 
a series concerned.””! 
1 From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life. 
