44 DEVELOPMENTAL AND ADAPTIVE CHANGES. 



wMcli I called attention in ray memoir " On tlie 

 development of ChJoi'on.."^ 



AYlien an animal quits tlie egg in a form unlike that 

 whicli it will present in its mature condition, the 

 changes througli which it passes in order to attain to 

 that final and mature state, are changes of simple 

 development. But, if during this period the creature 

 is leading a separate and independent life, it is evident 

 that it will be acted on also by external forces, and 

 will be liable, therefore, to gradual modification through 

 the action of natural selection ; such modifications 

 having reference to its then mode of life, and not to its 

 ultimate condition. These are obviously of an origin 

 distinct from the developmental changes ; they explain 

 many problems in natural history, which we cannot 

 otherwise understand, and I have proposed for them 

 the term " adaptational" or " adaptive." 



The larvge of insects are by no means mere stages 

 in the development of the perfect animal. On the 

 contrary, they undergo various changes and develop- 

 ments which have reference entirely to their own re- 

 quirements and condition. That this is so must, 

 indeed, be admitted by every believer in evolution. 

 External circumstances act on the animal throughout its 

 independent life, and consequently on larvae as well as 

 on perfect insects. It is evident, then, that while the 

 embryonic development of an animal in its mother's 

 womb, or in an egg, gives us an epitome of its specific 

 history (true as far as it goes, though certain parts 

 may be represented only by the briefest indications, or 

 even altogether omitted), this is by no means the 

 case with species in which the juvenile forms have a 

 separate and independent existence. External forces 

 and natural selection act on them as strongly as on the 

 mature forms. Hence, if an animal, which when young, 

 pursues one mode of life, and lives on one kind of food, 

 subsequently, either from its own growth in size and 



^ ' Trans. Linuean Society,' 1863 and 1865. 



