EMBRYOLOGY 255 



as yet known to be first developed under the nauplius- 

 form, though many appear as zoeas; nevertheless Miiller 

 assigns reasons for his belief, that if there had been no 

 suppression of development all these crustaceans would 

 have appeared as nauplii. 



How, then, can we explain these several facts in 

 embryology — namely, the very general, though not uni- 

 versal, difference in structure between the embryo and 

 the adult; — the various parts in the same individual em- 

 bryo, which ultimately become very unlike and serve for 

 diverse purposes, being at an early period of growth 

 alike; — the common, but not invariable, resemblance be- 

 tween the embryos or larvae of the most distinct species 

 in the same class; — the embryo often retaining, while 

 within the egg or womb, structures which are of no 

 service to it, either at that or at a later period of life; 

 on the other hand, larvse, which have to provide for 

 their own wants, being perfectly adapted to the surround- 

 ing conditions; — and lastly the fact of certain larvae 

 standing higher in the scale of organization than the 

 mature animal into which they are developed ? I believe 

 that all these facts can be explained, as follows. 



It is commonly assumed, perhaps from monstrosities 

 affecting the embryo at a very early period, that slight 

 variations or individual differences necessarily appear at 

 an equally early period. We have little evidence on this 

 head, but what we have certainly points the other way; 

 for it is notorious that breeders of cattle, horses, and 

 various fancy animals cannot positively tell, until some 

 time after birth, what will be the merits or demerits of 

 their young animals. We see this plainly in our own 

 children; we cannot tell whether a child will be tall or 



— SCIEXCE — 28 



