( iby 
abbreviation of the larval period may be of service: for 
instance, an army of caterpillars will have a great advantage 
if their period of feeding can be reduced, so that the eggs 
shall only require to be deposited when there is plenty 
of food ready for them, and yet their life shall not extend 
into the period when vegetation fails in vigour; clearly, too, 
defenceless caterpillars, without means of escaping from 
their numerous enemies, had better make this stage as short 
as possible. I think it will not be altogether lost time if we 
pursue this analysis a little farther: forif we do so I believe 
we shall find that abbreviation of the larval stage is to some 
extent connected with abbreviation of the duration of exist- 
ence in the perfect state. In order to make this clear, I 
must briefly recall to you the conditions under which the 
great functions of life are exercised in insects. In them the 
functions of growth and development are more completely 
separated in time than they are in the Vertebrata, and are 
more sharply divided from the period of reproduction: this is 
no doubt associated with—is possibly a specialization resulting 
from — certain remarkable peculiarities in their function 
of nutrition. In the Vertebrata waste and repair of the 
tissues are carried on by means of a single medium, the 
blood, which takes to the tissues at each moment, in the 
form of oxygen, the means for exercising function, that is 
deteriorating themselves, and the means for repairing that 
deterioration ; hence tear and repair go on pari passu. In 
the insect it is not so: the function of oxygenation is carried 
on by a separate medium, unconnected with the circulation 
of the blood ; hence waste can be carried on without repair. 
Turning now to the means of circulation of the blood, this 
is undoubtedly the weak point of the insect economy: there 
is no system for carrying the biood minutely into the tissues, 
and we have in the insect the curious phenomenon of indirect 
assimilation and the accumulation of unformed tissue to an 
enormous extent. These characters permit the phenomena 
of growth and development to be much separated, without 
being altogether independent. In the larval stage the 
tracheal system is less developed, and the accumulation of 
unformed tissue is carried to a most extraordinary extent; in 
