432 DEVELOPMENT AXD EMBRYOLOGY. 



insect, by Professor Huxley, we see hardly any trace of the 

 vermiform stage. 



Sometimes it is only the earlier developmental stages 

 which fail. Thus, Fritz Miiller has made the remarkable 

 discovery that certain shrimp-like crustaceans (allied to 

 Penoeus) first appear under the simple nauplius-form, and 

 after passing through two or more zoea-stages, and then 

 through the mysis-stage, finally acquire their mature struc- 

 ture : now in the whole great malacostracan order, to which 

 these crustaceans belong, no other member is 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 de- 

 velopment, all these crustaceans would have appeared as 

 nauplii. 



How, then, can we explain these several facts in embry- 

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

 difference 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 pur- 

 poses, being at an early period of growth alike ; the com- 

 mon, but not invariable, resemblance between 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, larvae which have 

 to provide for their own wants, being perfectly adapted to 

 the surrounding 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 af- 

 fecting the embryo at a very early period, that slight varia- 

 tions 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 and demerits of their young animals. 

 We see this plainly in our own children; we cannot tell 

 whether a child will be tall or short, or what its precise 

 features will be. The question is not, at what period of life 

 each variation may hav^ jueen caused, but at what period the 

 greets ty$ ^splayed, >Tte Q&j^j may frave a^ed, mi I 



