is rendered indispensable by the circumstance that a division of labour
has been set up between the various periods of life; for example, that
the larvae have exclusively taken upon themselves the business of
nourishment. A further circumstance to be taken into consideration is
the size of the eggs,--a simpler structure may be produced with less
material than a more compound one,--the more imperfect the larva, the
smaller may the egg be, and the larger is the number of these that the
mother can furnish with the same expenditure of material. As a rule, I
believe indeed, this advantage of a more numerous brood will not by any
means outweigh that of a more perfect brood, but it will do so in those
cases in which the chief difficulty of the young animals consists in
finding a suitable place for their development, and in which, therefore,
it is of importance to disperse the greatest possible number of germs,
as in many parasites.

As the conversion of the original development with metamorphosis into
direct development is here under discussion, this may be the proper
place to say a word as to the already indicated absence of metamorphosis
in fresh-water and terrestrial animals the marine allies of which still
undergo a transformation. This circumstance seems to be explicable in
two ways. Either species without a metamorphosis migrated especially
into the fresh waters, or the metamorphosis was more rapidly got rid of
in the emigrants than in their fellows remaining in the sea.

Animals without a metamorphosis would naturally transfer themselves more
easily to a new residence, as they had only themselves and not at the
same time multifarious young forms to adapt to the new conditions. But
in the case of animals with a metamorphosis, the mortality among the
larvae, always considerable, must have become still greater under new
than under accustomed conditions, every step towards the simplification