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 larv
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 the modification has been greater
in degree, and has taken a longer time to complete? I believe it has
thus been unconsciously used; and only thus can I understand the
several rules and guides which have been followed by our best
systematists. We have no written pedigrees; we have to make out
community of descent by resemblances of any kind. Therefore we choose
those characters which, as far as we can judge, are the least likely to
have been modified in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view
are as good as, or even sometimes better than, other parts of the
organisation. We
care not how trifling a character may be let it be the mere inflection
318