state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
or that of a species, the two sexes; and how enormously these sometimes
differ in the most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the males and
hermaphrodites of certain cirripedes, when adult, and yet no one dreams
of separating them. The naturalist includes as one species the several
larval stages of the same individual, however much they may differ from
each other and from the adult; as he likewise includes the so-called
alternate generations of Steenstrup, which can only in a technical sense
be considered as the same individual. He includes monsters; he includes
varieties, not solely because they closely resemble the parent-form, but
because they are descended from it. He who believes that the cowslip
is descended from the primrose, or conversely, ranks them together as
a single species, and gives a single definition. As soon as three
Orchidean forms (Monochanthus, Myanthus, and Catasetum), which had
previously been ranked as three distinct genera, were known to be
sometimes produced on the same spike, they were immediately included as
a single species. But it may be asked, what ought we to do, if it could
be proved that one species of kangaroo had been produced, by a long
course of modification, from a bear? Ought we to rank this one
species with bears, and what should we do with the other species?
The supposition is of course preposterous; and I might answer by the
argumentum ad hominem, and ask what should be done if a perfect kangaroo
were seen to come out of the womb of a bear? According to all analogy,
it would be ranked with bears; but then assuredly all the other species
of the kangaroo family would have to be classed under the bear genus.
The whole case is preposterous; for where there has been close descent
in common, there will certainly be close resemblance or affinity.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvae
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, though in these cases