Facts and Arguments for Darwin | Page 8

of these little, plump, whitish Isopods. In this way
I have examined thousands of them with the simple lens, and I have
also examined many hundreds with the microscope, without finding
any differences among the females, or any intermediate forms between
the two kinds of males.
To the old school this occurrence of two kinds of males will appear to
be merely a matter of curiosity. To those who regard the "plan of

creation" as the "free conception of an Almighty intellect, matured in
the thoughts of the latter before it is manifested in palpable, external
forms," it will appear to be a mere caprice of the Creator, as it is
inexplicable either from the point of view of practical adaptation, or
from the "typical plan of structure." From the side of Darwin's theory,
on the contrary, this fact acquires meaning and significance, and it
appears in return to be fitted to throw light upon a question in which
Bronn saw "the first and most material objection against the new
theory," namely, how it is possible that from the accumulation in
various directions of the smallest variations running out of one another,
varieties and species are produced, which stand out from the primary
form clearly and sharply like the petiolated leaf of a Dicotyledon, and
are not amalgamated with the primary form and with each other like the
irregular curled lobes of a foliaceous Lichen.
Let us suppose that the males of our Tanais, hitherto identical in
structure, begin to vary, in all directions as Bronn thinks, for aught I
care. If the species was adapted to its conditions of existence, if the
BEST in this respect had been attained and secured by natural selection,
fresh variations affecting the species as a species would be
retrogressions, and thus could have no prospect of prevailing. They
must rather have disappeared again as they arose, and the lists would
remain open to the males under variation, only in respect of their sexual
relations. In these they might acquire advantages over their rivals by
their being enabled either to seek or to seize the females better. The
best smellers would overcome all that were inferior to them in this
respect, unless the latter had other advantages, such as more powerful
chelae, to oppose to them. The best claspers would overcome all less
strongly armed champions, unless these opposed to them some other
advantage, such as sharper senses. It will be easily understood how in
this manner all the intermediate steps less favoured in the development
of the olfactory filaments or of the chelae would disappear from the
lists, and two sharply defined forms, the best smellers and the best
claspers, would remain as the sole adversaries. At the present day the
contest seems to have been decided in favour of the latter, as they occur
in greatly preponderating numbers, perhaps a hundred of them to one
smeller.

To return to Bronn's objection. When he says that "for the support of
the Darwinian theory, and in order to explain why many species do not
coalesce by means of intermediate forms, he would gladly discover
some external or internal principle which should compel the variations
of each species to advance in ONE direction, instead of merely
permitting them in all directions," we may, in this as in many other
cases, find such a principle in the fact that actually only a few
directions stand open in which the variations are at the same time
improvements, and in which therefore they can accumulate and become
fixed; whilst in all others, being either indifferent or injurious, they will
go as lightly as they come.
(FIGURE 7. Orchestia Darwinii, n. sp. male.)
The occurrence of two kinds of males in the same species may perhaps
not be a very rare phenomenon in animals in which the males differ
widely from the females in structure. But only in those which can be
procured in sufficient abundance, will it be possible to arrive at a
conviction that we have not before us either two different species, or
animals of different ages. From my own observation, although not very
extensive, I can give a second example. It relates to a shore-hopper
(Orchestia). The animal (Figure 7) lives in marshy places in the vicinity
of the sea, under decaying leaves, in the loose earth which the Marsh
Crabs (Gelasimus, Sesarma, Cyclograpsus, etc.) throw up around the
entrance to their borrows, and even under dry cow-dung and
horse-dung. If this species removes to a greater distance from the shore
than the majority of its congeners (although some of them advance very
far into the land and even upon mountains of a thousand feet in height,
such as O. tahitensis, telluris, and sylvicola), its male differs still more
from all known species by the powerful chelae