r.
Thus, a man and a worm are members of the animal kingdom in virtue of
certain apparently slight though really fundamental resemblances which
they present. But a man and a fish are members of the same sub-kingdom
'Vertebrata', because they are much more like one another than either of
them is to a worm, or a snail, or any member of the other sub-kingdoms.
For similar reasons men and horses are arranged as members of the
same Class, 'Mammalia'; men and apes as members of the same Order,
'Primates'; and if there were any animals more like men than they
were like any of the apes, and yet different from men in important
and constant particulars of their organization, we should rank them as
members of the same Family, or of the same Genus, but as of distinct
Species.

That it is possible to arrange all the varied forms of animals into
groups, having this sort of singular subordination one to the other, is
a very remarkable circumstance; but, as Mr. Darwin remarks, this is a
result which is quite to be expected, if the principles which he lays
down be correct. Take the case of the races which are known to be
produced by the operation of atavism and variability, and the conditions
of existence which check and modify these tendencies. Take the case
of the pigeons that I brought before you; there it was shown that
they might be all classed as belonging to some one of five principal
divisions, and that within these divisions other subordinate groups
might be formed. The members of these groups are related to one
another in just the same way as the genera of a family, and the groups
themselves as the families of an order, or the orders of a class;
while all have the same sort of structural relations with the wild
rock-pigeon, as the members of any great natural group have with a real
or imaginary typical form. Now, we know that all varieties of pigeons of
every kind have arisen by a process of selective breeding from a common
stock, the rock-pigeon; hence, you see, that if all species of animals
have proceeded from some common stock, the general character of their
structural relations, and of our systems of classification, which
express those relations, would be just what we find them to be. In other
words, the hypothetical cause is, so far, competent to produce effects
similar to those of the real cause.

Take, again, another set of very remarkable facts,--the existence of
what are called rudimentary organs, organs for which