I 



MUTUAL AFFINITIES OF ORGANIC BEINGS 399 



the development of this 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 structure: now in the whole great mala- 

 costracan order, to which these crustaceans belong, no other mem- 

 ber 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 develop- 

 ment, all these crustaceans would have appeared as nauplii. 



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

 namely, the very general, though not universal, difference in struc- 

 ture 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 common, 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, struc- 

 tures which are of no service to it, either at that or at a later 

 period of Hfe; on the other hand, larvae which have to provide for 

 their own wants, being perfectly adapted to the surrounding con- 

 ditions; 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 indi- 

 vidual 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 have been caused, but at what period the effects are dis- 

 played. The cause may have acted, and I believe often has acted, 

 on one or both parents before the act of generation. It deserves 

 notice that it is of no importance to a very young animal, as long 



