66 DOCTRINE OF EVOLUTION 



Like the fundamental principle of comparative anat- 

 omy in its sphere, the Law of Recapitulation, formu- 

 lated as a summary description of the foregoing and 

 similar facts, is one that holds true throughout the en- 

 tire range of embryology and for every division of the 

 animal series, however large or small. We have dis- 

 cussed its broader application, and now we may take 

 up some of the more or less special cases mentioned in 

 the earlier section of the present chapter, to see how it 

 may work in detail. 



The flounder was noted as a variant of the fish theme 

 which seemed to be a descendant of a symmetrical 

 ancestor because its structural plan was like that of 

 other bony fishes. If this be true, and if in its develop- 

 ment a flounder must review its mode of evolution 

 as a species, the young fish ought to be symmetrical; 

 and it actually is. The grotesque skate and hammer- 

 head shark were demonstrated to be derivatives of 

 a simpler type of shark ; their embryos are practically 

 indistinguishable from those of ordinary dogfish and 

 sharks. 



Among the jointed animals a wealth of mf^^resting 

 material is found by the embryologist. All crabs 

 seemed to be modified lobsterlike creatures ; to confirm 

 this interpretation, based solely upon details of adult 

 structure, young crabs pass through a stage when to all 

 intents and purposes they are counterparts of lobsters. 

 Even the twisted hermit crab, which has a soft-skinned 

 hinder part coiled to fit the curve of the snail shell 

 used as a protection, is symmetrical and lobster-like 

 when it is a larva. 



Among the insects many examples occur that are 



