220 



CHAP. 



opponent of the recapitulation theory could deny their ancestral 

 significance. 



It is a tacit assumption of the 





recapitulation theory that 

 advance in evolution is corre- 

 lated with a change in the 

 environment, and with the 

 consequent acquisition of 

 new kinds of food by the 

 animal in the adolescent 

 stage of its life-history. The 

 evidence that this theory is 

 well founded comes out more 

 and more strongly the more 

 the embryology of the 

 various members of the 

 animal kingdom is studied ; 

 but it is exceedingly difficult 

 to reconcile it with modern 

 work on the subject of 

 heredity, which appears to 

 point to the conclusion that 

 changes in morphology are 

 due to changes occurring in 

 the nuclear matter of the 

 germ cells before fertilization. 

 When such chemical changes 

 have been effected why, it 

 may be asked, should their 

 influence appear just at that 

 moment of development 

 when the environment is changed ? Light might be thrown on this 

 question by a careful and systematic study of the life-histories of 

 parasitic and abnormal forms, belonging to large families or orders in 

 the animal kingdom, which show well-established and stereotyped 

 normal features. Work on these questions will form one of the 

 most fascinating features of future embryological study. 



FIG. 168. Adult female of Portunion maenadis, 

 with appendages dissected out. (After Giard. ) 



ab, abdomen ; anf 1 , vestige of first antenna ; anf 2 , 

 vestige of second antenna ; ceph, swollen head ; mxp, 

 rudiment of maxillipede ; enp'*, the endopodite of the 

 second abdominal appendage ; enp$, the endopodite of the 

 third abdominal appendage ; oosl 1 , the three lobes of 

 the first oostegite of the right side ; cost 2 , the second 

 oostegite on the right side ; oost 3 , the third oostegite on 

 the right side ; oost-*, the fourth oostegite on the right 

 side ; oost 5 , the fifth oostegite on the right side. 



