LARVAL FORMS. 361 



secondary changes which may have occurred in the develop- 

 mental history of forms, which have either a long larval existence, 

 or which are born in a nearly complete condition, is primarily 

 determined by the nature of the favourable variations which can 

 occur in each case. 



Where the development is a fcetal one, the favourable varia- 

 tions which can most easily occur are (i) abbreviations, (2) an 

 increase in the amount of food-yolk stored up for the use of the 

 developing embryo. Abbreviations take place because direct 

 development is always simpler, and therefore more advantageous; 

 and, owing to the fact of the foetus not being required to lead an 

 independent existence till birth, and of its being in the mean- 

 time nourished by food-yolk, or directly by the parent, there are 

 no physiological causes to prevent the characters of any stage of 

 the development, which are of functional importance during a 

 free but not during a fcetal existence, from, disappearing from the 

 developmental history. All organs of locomotion and nutrition 

 not required by the adult will, for this reason, obviously have a 

 tendency to disappear or to be reduced in foetal developments; 

 and a little consideration will shew that the ancestral stages in 

 the development of the nervous and muscular systems, organs 

 of sense, and digestive system will be liable to drop out or be 

 modified, when a simplification can thereby be effected. The 

 circulatory and excretory systems will not be modified to the 

 same extent, because both of them are usually functional during 

 fcetal life. 



The mechanical effects of food-yolk are very considerable, 

 and numerous instances of its influence will be found in the 

 earlier chapters of this work 1 . It mainly affects the early stages 

 of development, i.e. the form of the gastrula, &c. 



The favourable variations which may occur in the free larva 

 are much less limited than those which can occur in the fcetus. 

 Secondary characters are therefore very numerous in larvae, and 

 there may even be larvae with secondary characters only, as, for 

 instance, the larvae of Insects. 



In spite of the liability of larvae to acquire secondary charac- 

 ters, there is a powerful counterbalancing influence tending 



1 For numerous instances of this kind, vide Chapter XI. of Vol. in. 



