This may be safely done, and is often done, as long as a sufficient number of characters, let them be ever so unimportant, betrays the hidden bond of community of descent.

Let two forms have not a single character in common, yet, if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class.

As we find organs of high physiological importance--those which serve to preserve life under the most diverse conditions of existence--are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification.

We shall presently see why embryological characters are of such high classificatory importance.

Geographical distribution may sometimes be brought usefully into play in classing large genera, because all the species of the same genus, inhabiting any distinct and isolated region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very important distinction between real affinities and analogical or adaptive resemblances.