Which of the different modes of development at present occurring in a
class of animals may claim to be that approaching most nearly to the
original one, is easy to judge from the above statements.

The primitive history of a species will be preserved in its
developmental history the more perfectly, the longer the series of young
states through which it passes by uniform steps; and the more truly, the
less the mode of life of the young departs from that of the adults, and
the less the peculiarities of the individual young states can be
conceived as transferred back from later ones in previous periods of
life, or as independently acquired.

Let us apply this to the Crustacea.


CHAPTER 12. PROGRESS OF EVOLUTION IN CRUSTACEA.

According to all the characters established in the last paragraph, the
Prawn that we traced from the Nauplius through states analogous to Zoea
and Mysis to the form of a Macrurous Crustacean appears at present to be
the animal, which in the section of the higher Crustacea (Malacostraca)
furnishes the truest and most complete indications of its primitive
history. That it is the most complete is at once evident. That it is the
truest must be assumed, in the first place, because the mode of life of
the various ages is less different than in the majority of the other
Podophthalma; for from the Nauplius to the young Prawn they were found
swimming freely in the sea, whilst Crabs, Porcellanae, the Tatuira,
Squilla, and many Macrura, when adult usually reside under stones, in
the clefts of rocks, holes in the earth, subterranean galleries, sand,
etc., not to mention other deviations in habits such as are presented by