58 THE GENESIS OF SrECIES. [Chap. 



the future color of llieir plumage," with many others. But 

 the idea that the modilication of any internal or external 

 part of the body 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 ellicient presence of an 

 unknown internal natural law or laws conditioning the evo- 

 lution of new specific forms from preceding ones, modified 

 by the action of surrounding conditions, by " Natural Se- 

 lection," and by other controlling influences. 



The same difficulty seems to present itself in* other ex- 

 amples 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 development 

 exists. The adult is not formed from the egg 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 en- 

 larges, and, having established a communication with the 

 exterior, envelops and appropriates the creature's stomach, 

 with which it swims away and develops into tlie complete 

 adult form, while the dispossessed individual perishes. 



Again, certain flies present a mode of develoi)ment 

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

 grul) is, as usual, produced from the ovum, but this grub, 

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

 undergoes a sort of liquefaction of a great part of its bodj', 

 while certain patches of formative tissue, which are attached 

 to the ramifying air-tubes, or tracheaj (and which patches 

 bear the name of "imaginal disks"), give rise to the legs, 

 wings, eyes, etc., respectively ; and these severally-formed 

 parts grow together, and build up the head and body by 

 their mutual approximation. Such a process is unknown 

 outside the class of insects, and inside that class it is only 



2i " Origin of Species," 5tli edit., 1869, p. 179. 



