INTRODUCTION 



23 



they are placed, it is thereby implied that they once possessed the 

 normal features exhibited by the typical members of these groups ; 

 and therefore, beyond all question, their late larval stages must be of 

 an ancestral character. 



It follows, then, that advances in evolution do, as a rule, manifest 

 themselves when the animal is fully adult. But recent research in 

 the laws of heredity has rendered it almost certain that inheritable 

 variations are only those which affect the nature of the germ cell, 

 and most zoologists refuse to believe that the adoption of a new mode 



B 



FIG. 8. The larva and adult female of 

 Portunion maenadis. (After Giard.) 



A, larva 1 just hatched. B, adult female, abd, ab- 

 domen ; afl, antennule ; a 2 , antenna ; br, brood-sac 

 composed of conjoined ovigerous plates of thorax ; g, 

 jaws or gnathites ; h, head ; pi, swimmerets or pleopods. 



of life by an adult could directly affect its germ cells. Lamarck's 

 idea that the change in the body induced by new habits could effect 

 a corresponding change in the germ cells is rejected by them. How, 

 then, is the adaptation effected ? 



If an animal assumes a new habit or mode of life with success, 

 this can only be because a new and abundant food supply is thereby 

 opened up to it. Now Darwin has suggested that a rich food supply 

 is the proximate cause of the arrival of variations. Hence we may 

 provisionally assume that the new food supply upsets the stability 

 of heredity by altering the chemical constitution of the hereditary 

 substance in the germ cells, and so variations in all directions are 



