INTEODUCTION 



25 



the record of the last habits assumed by the species, the main frame- 

 work of all developmental history must be the condensed record of 

 ancestral experience ; for each stage in the development of an animal 

 bears the same relationship to the one which immediately precedes it as 

 the adult stage does to the last larval stage. 



We must now consider some of the influences which modify the 

 ancestral character of developmental history. It is obvious that 

 there is no a priori impossibility that the supply of rich food should 

 produce variations which affect earher stages of the life-history than 



'Jl.>>^-I^^ 



pect 



Fig. 10.— The larva and first metamorphosed form of the Plaice [Plewronedes platessa). 



(After Cole and Johnstone. ) 



A, larva. B, young Plaice just after metamorphosis, an, anus ; pa(, pectoral fin. 



the adult stage, especially if the alterations produced thereby have a 

 "survival value." Thus we may get secondary adaptations ot the 

 larvae to their environment, which are seen to be secondary for the 

 reason that they differ widely from each other within the hmits ot 

 a group in which the adult structure is constant. When, for instance, 

 within the order of two-winged Flies we find some larvae adapted to 

 living in water, others to living in earth, others m dung, and still 

 others in dead bodies, and a few in hving bodies, we cannot regard 

 any of these methods of life as ancestral, and we find that in each 

 case the larva is specially altered to suit it to the special conditions 

 of its existence. 



