Decapoda, from the often incredible difference in the development of the
most nearly allied forms, these must be separately studied--usually
family by family, and frequently genus by genus--nay, sometimes, as in
the case of Peneus, even species by species; and because these
investigations, in themselves troublesome and tedious, often depend for
their success upon a lucky chance.

But although the satisfactory completion of the "Genealogical tree of
the Crustacea" appeared to be an undertaking for which the strength and
life of an individual would hardly suffice, even under more favourable
circumstances than could be presented by a distant island, far removed
from the great market of scientific life, far from libraries and
museums--nevertheless its practicability became daily less doubtful in
my eyes, and fresh observations daily made me more favourably inclined
towards the Darwinian theory.

In determining to state the arguments which I derived from the
consideration of our Crustacea in favour of Darwin's views, and which
(together with more general considerations and observations in other
departments), essentially aided in making the correctness of those views
seem more and more palpable to me, I am chiefly influenced by an
expression of Darwin's: "Whoever," says he ('Origin of Species' page
482), "is led to believe that species are mutable, will do a good
service by conscientiously expressing his conviction." To the desire
expressed in these words I respond, for my own part, with the more
pleasure, as this furnishes me with an opportunity of publicly giving
expression in words to the thanks which I feel most deeply to be due
from me to Darwin for the instructions and suggestions for which I am so
deeply indebted to his book. Accordingly I throw this sand-grain with
confidence into the scale against "the load of prejudice by which this