382 
F. M. BALFOUR. 
more advantageous ; and, owing to the fact of the foetus not 
being required to lead an independent existence till birth, 
and of its being in the meantime nourished by food-yolk, 
or directly by the parent, there are no physiological 
causes to prevent the characters of any stage of the develop- 
ment, which are only of functional importance to a free 
larva, from disappearing from the developmental history. 
All external organs of locomotion and nutrition will, for this 
reason, obviously have a tendency to disappear or to be reduced 
in foetal developments ; and a little consideration will show 
that the ancestral stages in the development of the nervous 
and muscular systems, organs of sense, and digestive system 
will be liable to drop out or be modified, when a simplifica- 
tion can thereby be affected. The circulatory and excretory 
systems will not be modified to the same extent, because both 
of them are usually functional during foetal life. 
The mechanical efiects of food-yolk are very considerable, 
and numerous instances of their influence will be found in 
my treatise on ^ Comparative Embryology.^ They mainly 
affect the early stages of development, i.e. the form of the 
gastrula, &c. 
The favorable variations which can occur in the free 
larva are much less limited than those which can occur in the 
foetus. Secondary characters are therefore very numerous 
in larvse, and there may even be larvae with secondary 
characters only, as, for instance, the larvae of insects. 
In spite of the liability of larvae to acquire secondary 
characters, there is a powerful counter-balancing influence 
tending towards the preservation of ancestral characters, in 
that larvae are necessarily compelled at all stages of their 
growth to retain in a functional state such systems of organs, 
at any rate, as are essential for a free and independent 
existence. It thus comes about that in spite of the many 
causes tending to produce secondary changes in larvae, there 
is always a better chance of their repeating, in an unabbre- 
viated form, their ancestral history than is the case with 
embryos, which undergo their development within the egg. 
It may be further noted as a fact which favours the re- 
lative retention by larvae of ancestral characters, that a 
secondary larval stage is less likely to be repeated in de- 
velopment than an ancestral stage, because there is always 
a strong tendency for the former, which is a secondarily inter- 
calated link in the chain of development, to drop out by the 
occurrence of a reversion to the original type of development. 
The relative chances of the ancestral history being pre- 
served in the foetus or the larva may be summed up in the 
