LARVAL FORMS. 363 



Types of Larvae. Although there is no reason to suppose 

 that all larval forms are ancestral, yet it seems reasonable to 

 anticipate that a certain number of the known types of larvae 

 would retain the characters of the ancestors of the more im- 

 portant phyla of the animal kingdom. 



Before examining in detail the claims of various larvae to 

 such a character, it is necessary to consider somewhat more at 

 length the kind of variations which are most likely to occur in 

 larval forms. 



It is probable a priori that there are two kinds of larvae, 

 which may be distinguished as primary and secondary larvae. 

 Primary larvae are more or less modified ancestral forms, which 

 have continued uninterruptedly to develop as free larvae from 

 the time when they constituted the adult form of the species. 

 Secondary larvae are those which have become introduced into 

 the ontogeny of species, the young of which were originally 

 hatched with all the characters of the adult; such secondary 

 larvae may have originated from a diminution of food-yolk in 

 the egg and a consequently earlier commencement of a free 

 existence, or from a simple adaptive modification in the just 

 hatched young. Secondary larval forms may resemble the 

 primary larval forms in cases where the ancestral characters were 

 retained by the embryo in its development within the egg; but 

 in other instances their characters are probably entirely adaptive. 



Causes tending to produce secondary changes in larv<z. The 

 modes of action of natural selection on larvae may probably be 

 divided more or less artificially into two classes. 



1. The changes in development directly produced by the 

 existence of a larval stage. 



2. The adaptive changes in a larva acquired in the ordinary 

 course of the struggle for existence. 



The changes which come under the first head consist essen- 

 tially in a displacement in the order of development of certain 

 organs. There is always a tendency in development to throw 

 back the differentiation of the embryonic cells into definite 

 tissues to 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 abbreviated way its phylogenetic history, 

 to be effected with the least expenditure of energy. Owing to 



