298 F(ETAL AND LARVAL DEVELOPMENT. 



adult, and reach the adult state either by a series of small steps, or 

 by a more or less considerable metamorphosis. 



The satisfactory application of embryological data to morphology 

 depends upon a knowledge of the extent to which the record of 

 ancestral history has been preserved in development. Unless 

 secondary changes intervened this record would be complete ; it 

 becomes therefore of the first importance to the embryologist to 

 study the nature and extent of the secondary changes likely to occur 

 in the fffital or the larval state. 



The principles which govern the perpetuation of variations which 

 occur in either the larval or the fcetal state are the same as those 

 for the adult condition. Variations favourable to the survival of the 

 species are equally likely to be perpetuated, at whatever period of 

 life they occur, prior to the loss of the reproductive powers. The 

 possible nature and extent of the secondary changes which may have 

 occurred in the developmental history of forms, which have either a 

 long larval existence, or which are bom in a nearly complete con- 

 dition, 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 variations 

 which can most easily occur are — (1) 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 meantime nourished by food-yolk, 

 or directly by the parent, there are no physiological causes to pre- 

 vent the characters of any stage of the development, which are of 

 functional importance dui^ing a free hut not during a foetal 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 he effected. The 

 circulatory and excretory systems will not be modified to the same 

 extent, because both of them are usually functional during foetal 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\ It mainly affects the early stages of de- 

 velopment, 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 foetus. 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. 



' For numerous instances of this kind, vide Chapter xi. of Vol. ii. 



