II.] INCIPIE::T STRUCTURES. 51 



simultaneously affected, and increase proportionally, or pos- 

 sibly so decrease. Examples of such are the hair and. teeth 

 in the naked Turkish dog, the general deafness of white 

 cats with blue eyes, the relation between the presence of 

 more or less down on younc]^ birds when first hatched, and. 

 the future colour of their plumage,^ with many others. 

 But the idea that the modification of any internal or exter- 

 nal part of the l>ody of an Echinus carries with it the 

 effect of producing elongated, flexible, triradiate, snapping 

 processes, is, to say the very least, fully as obscure and 

 mysterious as what is here contended for, viz. the efficient 

 presence of an unknown internal natural law or laws condi- 

 tioning the evolution of new specific forms from preceding 

 ones, modified by the action of surrounding conditions, by 

 '"Xatural Selection" and l)y other controlling influences. 



The same difficulty seems to present itself in other 

 examples of exceptional structure and action. In the 

 same Echinus, as in many allied forms, and also in some 

 more or less remote ones, a very peculiar mode of develop- 

 ment exists. The adult is not formed from the Qgg directly, 

 but the egg gives rise to a creature which swims freely 

 about, feeds, and is even somewhat complexly organized. 

 Soon a small lump appears on one side of its stomach; this 

 enlarges, and, having established a communication with the 

 exterior, envelopes and appropriates the creature's stomach, 

 with which it swims away and develops into the complete 

 adult form, while the dispossessed individual perishes. 



Again, certain flies present a mode of development 

 equally bizarre, though quite different. In these flies, the 

 grub is, as usual, produced from the ovum ; but this grub, 

 instead of growing up into the adult in the ordinary way, 



1 "Origin of Species," 5th edit. 1869, p. 179. 



E 2 



