CHAP. XII. EVOLUTION IN CRUSTACEA. 131 



of, as elsewhere, downwards. Does not so isolated a 

 phenomenon as this, it might be asked, in the sense of 

 Darwin's theory, indicate a common inheritance ? Does 

 it not necessitate that we should unite as the descend- 

 ants of the same primitive ancestors, Mysis with the 

 Isopoda on the one hand, and on the other the rest of 

 the Podophthalma with the Amphipoda ? I think not. 

 Such a necessity exists only for those who estimate a 

 peculiarity at a higher value because it makes its ap- 

 pearance at an earlier period of the egg-life. Whoever 

 regards species as not created independently and un- 

 changeably, but as having gradually become what they 

 are, will say to himself that, when the ancestors of our 

 Mysides came (probably much later than those of the 

 Amphipoda and Isopoda) to develope numerous body- 

 segments and limbs whilst still embryos, as they could 

 no longer find room in the egg when extended straight 

 out, and were therefore compelled to bend themselves, 

 this could only take place either upwards or down- 

 wards, and whatever conditions may have decided the 

 direction actually adopted, any near relationship to 

 either of the two orders of Edriophthalma could hardly 

 have taken part in it. 



It may, however, be remarked, that the different cur- 

 vature of the embryo in the Amphipoda and Isopoda is so 

 far instructive, as it proves that their present mode of de- 

 velopment was adopted only after the separation of these 

 orders, and that, in the primitive stock of the Edrioph- 

 thalma, the embryos were, if not Nauplii, at least short 

 ., enough in the body to find room in the egg in an 



K 2 



