AND THE LOWER ANIMALS. 119 



insufficiency of the organizable materials of the 

 vitellus as an approximate cause of the phenomena 

 we have under consideration, we can explain, or 

 rather co-ordinate, facts which, apart from this view, 

 we could not associate together. The greater this 

 insufficiency is, the more imperfect will the embryo 

 be which is formed at the expense of the vitellus, 

 the more removed will it be from its final characters, 

 and the greater must be the supply with which it is 

 furnished in approaching to and attaining them. 

 Observation bears out this conclusion. Insects' eggs 

 are enormous when compared with those of certain 

 mollusks. The egg of Gossus ligniperda is about 

 thirty thousand times larger than that of the Teredo. 

 Moreover, the little caterpillar which springs from 

 it is already a very complex animal, or, in other words, 

 a very advanced embryo. The Teredo, on the other 

 hand, is in the first instance as simple as possible. 

 Its body is, so to speak, but a homogeneous pulp, 

 in which a digestive tube is vaguely discernible. The 

 caterpillar will, doubtless, have to form some organs, 

 but more especially to develop and modify those 

 which it has got already : the Teredo will have to 

 acquire all its parts. 



We have pointed out the nature, and at least one, 

 of the chief causes of metamorphosis, and of its 

 modifications.* Is it necessary to dwell upon the 



parede has expressed the same idea, in summing up the views pro- 

 pounded by Leuckart. These are his words : — " What does meta- 

 morphosis consist in ? ... It is the passage of an animal deprived 

 of its egg-envelope through phases whose object is to bring it to 

 its own or its parent's type, and which among other creatures takes 

 place within the egg itself." 



* In 1855 I said reservedly, " It is not my intention to lay down 



