398 DEVELOPMENT Chap. XIII. 



pass throuu'li a worm-like sta_o;o, Avhcthcr they arc active and 

 adapted to diversified habits, or are inactive from lieing placed 

 in the midst of proper nutriment or from bcinpf fed by their 

 parents ; but in some few cases, as in that of Aphis, if we look 

 to the admiraljle drawings of the development of this insect, by 

 Prof. Huxley, wc sec hardly any trace of the vermiform stage. 



Sometimes it is only the earlier developmental stages 

 which fail. Thus Fritz Miiller has made the remarkable dis- 

 covery tliat certain shrimp-like crustaceans (allied to Penoeus) 

 first appear imder the simple nauplius-form, and after passing 

 through two or more zoea-stages, and then through the mysis- 

 stage, finally acquire their mature structure : now in the Avhole 

 great malacostracan class, to which these crustaceans belong, 

 no other member is as yet known to be first developed under 

 the nau})lius-form, though many appear as zoeas; nevertheless, 

 Miiller assigns reasons for his belief that all these crustaceans 

 would luiA-c appeared as nauplii, if tliere had been no suppres- 

 sion of development. 



How, then, can we explain these several facts in embryology 

 — namely, the very general, though not universal, diflerence 

 in structure between the embryo and the adult — the various 

 parts in the same individual embryo, which ultimately become 

 very unlike and serve for diverse purposes, being at an early 

 period of growth alike — the general, but not invariable, resem- 

 blance between the embryos or larvre of the most distinct 

 species in the same class — the embryo retaining, Avhile within 

 the egg or womb, structures which are of no service to it, 

 either at that period or later in life ; Avhile embryos at a later 

 ]icriod, or larvie, which have to provide for their own wants, 

 are perfectly adapted to the surrounding conditions — and lastly 

 the fact of certain larvas standing higher in the scale of organi- 

 zation than the mature animals into which they are developed ? 

 I believe tliat all these facts can be explained, as follows : 



It is commonly assumed, perhaps from monstrosities affect- 

 ing the embryo at a very early period, that slight variations or 

 individual dilferences necessarily appear at an equally early 

 l)eriotl. ^Vc have little evidence on this head, but what we 

 liave certainly points the other way ; for it is notorious that 

 l)recders of cattle, horses, and various fancy animals, cannot 

 positively tell, until "some time after birth, what the merits or 

 form of their young animals will turn out. We see this plainly 

 in our own children ; wc cannot tell whether a child Avill be 

 tall or short, or what its jirecise features will be. The ques- 



