in the family of the Tanaidae.* (* Whether the want of the abdominal
feet in the young of Tanais be an inheritance from the time of the
primitive Isopoda, or a subsequently acquired peculiarity, which appears
to me the more admissible view at present, may perhaps be decided with
some certainty, when we become acquainted with the development and mode
of life of its family allies, Apseudes and Rhoea. The latter, as is well
known, is the only Isopod which possesses a secondary flagellum on the
anterior antennae. I have recently obtained a new and unexpected proof
that the Tanaidae ("Asellotes heteropodes" M.-Edw.) of all known
Crustacea approach most closely to the primitive form of the
Edriophthalma. Mr. C. Spence Bate writes to me: "Apseudes, as far as I
know, is the ONLY Isopod in which the antennal scale so common in the
Macrura is present on the lower antenna.") If any one will furnish me
with an Amphipod or an Isopod with Nauplius-brood, the existence of
which would not be more remarkable in independently produced species
than that of a Prawn with Nauplius-brood, I will abandon the whole
Darwinian theory.

With regard to the Crabs, and also to the Isopoda and Amphipoda, we were
led to the assumption that, about the period when these groups started
from the common stem, a simplification of their process of development
took place. This also seems to be intelligible from Darwin's theory.
When any circumstances favourable to a group of animals caused its wider
diffusion and divergence into forms adapting themselves to new and
various conditions of existence, this greater variability, which betrays
itself in the production of new forms, will also favour the
simplification of the development which is almost always advantageous,
and moreover, exactly at this period, during adaptation to new
circumstances, as has already been indicated with regard to fresh-water
animals, this simplification will be doubly beneficial, and therefore,