XV 



DEVELOPMENT 



317 



vivid idea of development as progress from the general 

 to the special, we must be particularly careful to notice 

 that he did not say that the young mammal was once 

 like a little fish, afterwards like a reptile, and so on ; he 

 compared the embryo mammal at one stage with the 



FIG. 104. EMBRYOS OF FOWL, a ; DOG, b ; MAN, c. 

 (From Chambers's Encyclop. ; after Haeckel.) 



embryo fish, at another stage with the embryo reptile, 

 which is a very different matter. 



Fritz Miiller, in his Facts for Darwin, illustrated the 

 idea of recapitulation in his studies of the life-histories of 

 Crustacea. When a young crayfish is hatched, it is 

 practically a miniature adult. When a young lobster 

 is hatched, it differs not a little from the adult, and is 

 described as being at a Mysis stage, Mysis being a 

 prawn-like crustacean. It grows and moults and becomes 

 a little lobster. When a crab is hatched, it is quite un- 

 like the adult, it is liker one of the humblest Crustacea 

 such as the common water-flea Cyclops, and is described 

 as a Zosea. This Zosea grows and moults, and becomes, 

 not yet a crab, but a prawn-like animal with extended 

 tail, a stage known as the Megalopa. This grows and 

 moults, tucks in its tail, and becomes a young crab. And 

 again, w r hen the shrimp-like crustacean, known as Penceus, 

 is hatched, it is simpler than any known crustacean, it is 

 an unsegmented somewhat shield-shaped little creature 

 with three pairs of appendages and a median eye. It is 

 known as a Nauplius and resembles the larvae of most of 

 the lower crustaceans. It grows and moults and becomes 



