rm of the lower Crustacea, and through which the
greater part of the Crabs, Shrimps, Barnacles, water fleas, etc., pass
to attain their definite adult condition. A little water flea was
described as a separate genus, Nauplius, before it was known to be the
larva of a higher water flea, and so also Leptus was thought to be a
mature mite. Accordingly, we follow the usage of certain naturalists in
dealing with the Crustacea, and propose for this common primitive larval
condition of insects the term Leptus.

[Illustration: 190. Zoea.]

The first to discuss this subject of the ancestry of insects was Fritz
Mueller, who in his "Fuer Darwin,"[15] published in 1863, says, at the end
of his work, "Having reached the Nauplius, the extreme outpost of the
class, retiring farthest into the gray mist of primitive time, we
naturally look round us to see whether ways may not be descried thence
towards other bordering regions. * * * But I can see nothing certain.
Even towards the nearer provinces of the Myriopoda and Arachnida I can
find no bridge. For the Insecta alone, the development of the
Malacostraca [Crabs, Lobsters, Shrimps, etc.] may perhaps present a
point of union. Like many Zoeae, the Insecta possess three pairs of limbs
serving for the reception of nourishment, and three pairs serving for
locomotion; like the Zoeae they have an abdomen without appendages; as in
all Zoeae the mandibles in Insecta are destitute of palpi. Certainly but
little in common, compared with the much which distinguishes these two
animal forms. Nevertheless, the supposition that the Insecta had for
their common ancestor a Zoea which raised itself into a life on land,
may be recommended for further examination" (p. 140).

Afterwards Haeckel in his "Generelle Morphologie" (1866) and "History of
Creation," published in 1868, reiterates the notion that the insects are
derived from the larva (Zoea, Fig. 190) of the crabs, though he is
doubtful whether they did not originate directly from the worms.[16]

It may be said in opposition to the view that the insects came
originally from the same early crustacean resembling the larva of a crab
or shrimp, that the differences between the two types are too great, or,
in other words, the homologies of the two classes too remote,[17] and
the two types are each too specialized to lead us to suppose that one
was derived from the other. Moreover, we find through the researches of
Messrs. Hartt and Scudder that there w