XXI.] 



EilBKYOLO(JY. 269 



member is yet known to be developed under the nauplius 

 form, though very many appear as zoeas." ^ The E"auplius 

 resembles the larva of the Cirrhipedes ; and Darwin agrees 

 with Fritz Miiller that a similar form was the ancestor of 

 the whole crustacean class.^ It is another most remarkable Fresh- 

 fact, that the fresh-water Crustacea pass through no meta- Crustacea 

 morphosis at all ; their mature forms are developed du-ectly undergo 

 from the egg.^ I shall have to speak of the probable morphosis. 

 significance of this fact further on. 



Supposing it to be true that the larva or embrj^o is a 

 picture of what the ancestral form of the species was, 

 these facts must be accounted for by supposing that there 

 has been a substitution of direct development for indirect ; Direct de- 

 er, in other words, that one or more stages of the process Jubstr^'^ 

 of development have been left out. I mean that, when a tuted for 

 particular course of metamorphosis is characteristic of a 

 class, but is not found in all the members of the class, 

 it is most probable that those species which are developed 

 directly without metamorphosis are descended from others 

 which passed through the metamorphosis. Such a change Laws of 

 is consistent with what we know of the laws of Habit. ^ ^ 

 In the chapter on that subject we have seen that it is a 

 common case for an inherited variation to appear in the 

 offspring, not at birth, but at the same age at which it 

 first appeared in the parent ; but that it sometimes appears 

 earlier in the offspring. Now, if it is true that the embryo 

 is a picture of the ancestral form, and that the mature 

 form is descended from an ancestor resembling the em- 

 bryonic or larval one, it follows, to use Darwin's words, explaining 

 that "the adult differs from the embryo [or larva], owing pj^^g^"^" 

 to variations supervening at a not early age, and being 

 inherited at a corresponding age."* But if, from some 

 spontaneous variation, the offspring inherits and manifests and the 

 the variation in question, not at the corresponding age ^g^amoj.. 

 but at birth, this will amount to leaving out the first phosis. 



1 Darwin's Origin of Species, p. .523. 2 ibid. p. 531. » Ibid. p. 522. 



* Ibid. p. 406. I make this quotation, not as a testimony to a fact, 

 for this can be only matter of inference, but a-s showing that I agree 

 ■with Darwin's opinion on this part of the question. 



