THE ARGUMENT FROM EMBRYOLOGY 89 



ished in size, and become rudimentary. Crabs, 

 however, in their early stages of development, are 

 free-swimming animals, and have tails quite as well 

 developed and fully as large, relatively to the whole 

 animal, as lobsters ; and it is only after they have 

 reached a certain size that they abandon their free- 

 swimming habits, sink to the bottom, and henceforth 

 move by walking only. The case is exactly parallel 

 to that of the flat fish ; and the Recapitulation 

 theory explains the developmental history of a crab 

 by saying that it is a repetition of the ancestral 

 history of crabs in general : that crabs are descended 

 from animals essentially similar to lobsters — i.e., 

 from Macrurous ancestors, and that each crab 

 passes through a lobster stage in its develop- 

 ment, because of the inherited tendency that all 

 animals have to climb up their own genealogical 

 trees. (Fig. 13.) 



The evidence of the descent of crabs from Macru- 

 rous ancestors involves, moreover, the supposition 

 that they came into existence later than the Macrura. 

 This supposition is supported by the evidence of 

 palaeontology, for the Macrura are found as fossils 

 in the Devonian and Carboniferous periods, and 

 abundantly so in the Jurassic and Cretaceous, but 

 are comparatively scanty in the Tertiary period ; the 

 Brachyura, or crabs, on the other hand, are very 

 abundant in the Eocene and numerous in the 

 Cretaceous, but doubtfully represented in the earlier 

 periods. 



Good examples of recapitulation are found in 

 Molluscs. The typical Gasteropod has a large 



