300 CHANGES IN LARVjE. 



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 larvce. — 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 struofgle for existence. 



The changes which come under the first head consist essentially 

 in a displacement in the order of development of certain organs. 

 There is always a tendency in development to throw back the 

 dift'erentiation 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 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 foetal life is shortened, an earlier differentiation 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 complete differentiation of many organs 

 being retarded relatively to the muscular, nervous, and digestive 

 systems. 



The possible changes under the second head appear to be un- 

 limited. There is, so far as I see, no possible 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 



