78 ON THE ORIGIN AND [CHAP. 



for biting and chewing leaves. The same reasoning 

 applies also to the digestive organs. Hence the 

 caterpillar undergoes little, if any, change, except 

 in size, and the metamorphosis is concentrated, so 

 to say, into the last two moults. The changes then 

 become so rapid and extensive, that the interme- 

 diate period is necessarily one of quiescence. In 

 some exceptional cases, as in Sitaris (ante, p. 30) we 

 even find that, the conditions of life not being 

 uniform throughout the larval period, the larva itself 

 undergoes metamorphoses. 



Owing to the fact that the organs connected with 

 the reproduction of the species come to maturity at a 

 late period, larvae are generally incapable of breeding. 

 There are, however, some flies which have viviparous 

 larvae, and thus offer a typical case of alternation of 

 generations. 



Thus, then, we find among insects every gradation, 

 from simple growth to alternation of generations ; and 

 see how, from the single fact of the very early period 

 of development at which certain animals quit the 

 egg, we can throw some light on their metamorphoses, 

 and for the still more remarkable phenomenon that, 

 among many of the lower animals, the species is 

 represented by two very different forms. We may 

 even conclude, from the same considerations, that this 

 phenomenon may in the course of ages become still 

 more common than it is at present. As long, however, 

 as the external organs arrive at their mature form 

 before the internal generative organs are fully de- 

 veloped, we have metamorphosis ; but if the reverse is 

 the case, then alternation of generations often results. 



