1 66 MARINE BIOLOGICAL LABORATORY. 



acted upon in an increased degree, there seems every 

 reason to believe that the young may have been much 

 more acted upon and suffered far greater changes. 



On the other hand, when we see in such a group as 

 the Vertebrates that in the higher forms the young 

 have been removed to a large extent from the action 

 of surrounding conditions, — as, for instance, by being 

 enclosed within a shell as in the Sauropsida, or retained 

 within" the uterus in mammals, — then can we under- 

 stand why the young resemble each other more closely 

 than do the adults, for the obvious reason that the 

 adults have had to adapt themselves to more numerous 

 external conditions while the embryo has remained fixed. 



Indeed this may be pushed a step further, it seems to 

 me, and explain why such young retain the characteristics 

 of lower forms while the adults have lost such struc- 

 tures. This may be due to the young having been 

 removed to a greater extent than the adults from a 

 process of active selection. Hence in such a group, 

 when we say that the Ontogeny tends to repeat the 

 Phylogeny, we mean that the embryos have retained 

 more of the ancestral features than have the adults. 



But in such groups as the ones we are discussing, — 

 Annelids, Crustacea, etc., — we ought to expect, if what 

 I have said be true, the reverse of what we find in such 

 a group as the higher vertebrates ; viz. that the young 

 forms diverge far apart, and the adults come nearer 

 together. 



This will tell strongly against the position taken by 

 Hoek (also against the Nauplius theory), and render 

 unnecessary or even improbable that we need bring 

 together such forms as the Trochophore of the Annelids 



