us to descend into the details of their structure, must in the same
proportion bear in itself the warrant of its truth, and the more
convincingly prove that the foundation upon which it is built is no
loose sand, and that it is more than merely "an intellectual dream."

3. In the third place, however, it was possible, and this could not but
appear, prima facie, the most probable case, that the attempt might be
frustrated by the difficulties standing in its way, without settling the
question, either way, in a perfectly satisfactory manner. But if it were
only possible in this way to arrive for oneself at a moderately certain
independent judgment upon a matter affecting the highest questions so
deeply, even this alone could not but be esteemed a great gain.

Having determined to make the attempt, I had in the first place to
decide upon some particular class. The choice was necessarily limited to
those the chief forms of which were easily to be obtained alive in some
abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the
Diastylidae, the Amphipoda and Isopoda, the Ostracoda and Daphnidae, the
Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast,
representing the class of Crustacea with the deficiency only of the
Phyllopoda and Xiphosura, furnished a long and varied, and at the same
time intimately connected series, such as was at my command in no other
class. But even independently of this circumstance the selection of the
Crustacea could hardly have been doubtful. Nowhere else, as has already
been indicated by various writers, is the temptation stronger to give to
the expressions "relationship, production from a common fundamental
form," and the like, more than a mere figurative signification, than in
the case of the lower Crustacea. Among the parasitic Crustacea,
especially, everybody has long been accustomed to speak, in a manner
scarcely admitting of a figurative meaning, of their arrest of