384 
F, M. BALFOUR. 
Causes tending to produce secondartj changes in larm . — 
The ways in which natural selection can act on larvae may 
probably be divided more or less artificially into two classes. 
1. The changes in development necessarily produced by 
the existence of a larval stage. 
The adaptive changes in a larva acquired in the ordi- 
nary course of the struggle for existence. 
The changes which come under the first head consist 
essentially in a displacement in the order of deve- 
lopment of certain organs. There is always a tendency in 
development to throw back the differentiation of the embry- 
onic cells into definite tissues till as late a date as possible. 
This takes place in order to enable the changes of form, 
which every organ undergoes in repeating even in an abbre- 
viated way its phylogenetic history, to be effected with the 
least expenditure of energy. Owing to this tendency it 
comes about that when an organism is hatched as a larva 
many of the organs are still in an undifferentiated state, 
although the ancestral form which this larva represents 
had all its organs fully differentiated. In order, however, 
that the larva may be enabled to exist as an independent 
organism certain sets of organs, e.g. the muscular, nervous, 
and digestive systems, have to be histologically differentiated. 
If the period of hatching becomes earlier, an earlier differen- 
tiation of certain organs is a necessary consequence ; and in 
almost all cases the existence of a larval stage causes a 
displacement in order of development of organs, the com- 
plete differentiation of many organs being retarded relatively 
to the muscular, nervous, and digestive systems. 
The possible changes under the second head appear to 
be unlimited. There is, so far as I see, no possibJe reason 
why an indefinite number of organs should not be developed 
in larvae to protect them from their enemies, and to enable 
them to compete with larvae of other species, and so on. The 
only limit to such development appears to be the shortness of 
larval life, which is not likely to be prolonged, since, ceteris 
paribus j the more quickly maturity is reached the better it is 
for the species. 
A very superficial examination of marine larvae shows that 
there are certain peculiarities common to most of them, and 
it is important to determine how far such peculiarities are to 
be regarded as adaptive. Almost all marine larvae are provided 
with well-developed organs of locomotion, and transparent 
bodies. These two features are precisely those which it 
is most essential for such larvae to have. Organs of locomo- 
tion are important, in order that larvae may be scattered as 
