development by parasitism, as if the transformation of species were a
matter of course. It would certainly never appear to any one to be a
pastime worthy of the Deity, to amuse himself with the contrivance of
these marvellous cripplings, and so they were supposed to have fallen by
their own fault, like Adam, from their previous state of perfection.

That a great part of the larger and smaller groups into which this class
is divided, might be regarded as satisfactorily established, was a
further advantage not to be undervalued; whilst in two other classes
with which I was familiar, namely, the Annelida and Acalephae, all the
attempted arrangements could only be considered preliminary revisions.
These undisplaceable groups, like the sharply marked forms of the hard,
many-jointed dermal framework, were not only important as safe starting
points and supports, but were also of the highest value as inflexible
barriers in a problem in which, from its very nature, fancy must freely
unfold her wings.

When I thus began to study our Crustacea more closely from this new
stand-point of the Darwinian theory,--when I attempted to bring their
arrangements into the form of a geological tree, and to form some idea
of the probable structure of their ancestors,--I speedily saw (as indeed
I expected) that it would require years of preliminary work before the
essential problem could be seriously handled. The extant systematic
works generally laid more weight upon the characters separating the
genera, families and orders, than upon those which unite the members of
each group, and consequently often furnished but little employable
material. But above all things a thorough knowledge of development was
indispensable, and every one knows how imperfect is our present
knowledge of this subject. The existing deficiencies were the more
difficult to supply, because, as Van Beneden remarks with regard to the