THE ARGUMENT FROM EMBRYOLOGY 105 



one of gradual progression, and not one of abrupt 

 metamorphosis, each step being a step onwards 

 towards the adult insect. In the cockroach, again, 

 the process of development is gradual, and parts not 

 present in the larva, such as the wings, appear not 

 suddenly, but step by step and progressively. The 

 differences in these cases between the young and the 

 adult are not great. 



There is no doubt that gradual transformation is 

 the simpler and more primitive condition, and that 

 the action of Natural Selection may cause the larval 

 and adult forms to move apart and constitute periods 

 of growth and reproduction respectively. Natural 

 Selection may also cause such specialisation of the 

 adult condition as to make it differ widely from the 

 larva in habits and the nature of its food as well as 

 in structure, and the larva and adult may move so far 

 apart as to render a period of quiescence necessary 

 in order to allow the change of organs into those 

 required for new work. Again, Natural Selection may 

 act on the larva as well, fitting it better for its own 

 life as distinct from that of the imago ; thus leading 

 to still further divergence, and resulting in the larva 

 acquiring characters that are no part of its ancestral 

 history. 



A similar explanation applies to the process 

 whereby the young sea-urchin is formed within the 

 larva. The larva is adapted for a free-swimming 

 existence, the sea-urchin for crawling on the sea 

 bottom. A gradual transformation from one to the 

 other would be undesirable, for the intermediate con- 

 ditions would be imperfectly adapted to either mode 



