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