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