412 Dr. F. Mu'llcr on the Darwinian Hypothesis 



on the contrary, observation proved that all these terrestrial 

 species present the same modification of the respiratory appa- 

 ratus, the Darwinian theory could only account for them by 

 assuming that these terrestrial species belonging to various 

 families, which we may designate as a*, b t , c*, &c, descended 

 directly from a common type, T, which had already acquired the 

 organic conditions of aerial respiration. But then the theory 

 would contradict itself ; for whilst the study of the respiratory 

 organs would compel us to make «*, 6', c*, &c. descend from T, 

 the examination of the distinctive characters of the families 

 leads us to assign to each of these types a different origin, as it 

 makes a 1 descend from A, 6* from B, and c t from C. 



The details of the organization of the respiratory apparatus 

 in the land Crabs have hitherto been unknown; and thus a fine 

 field of investigation was open for Dr. Midler. If he found in 

 the terrestrial species of different families the same arrangement 

 for effecting aerial respiration, the Darwinian theory would be 

 irrevocably condemned ; but if he should discover differences so 

 complete as not to be reducible to the same type, this would 

 certainly furnish a strong argument in favour of the theory : 

 and the latter alternative has proved to be the true one. 



In an Aratus which climbs upon the branches of the man- 

 groves, and in a Grapsus which runs about the rocks of Santa 

 Catharina, the air finds entrance to the branchial cavity by a 

 fissure situated above the last pair of feet. These Crabs open 

 this respiratory fissure by elevating the posterior extremity of 

 the carapace. This aperture is consequently at the extremity of 

 the branchial cavity opposite to that by which water enters and 

 issues; for the apertures for the ingestion and egestion of water 

 are in the same position in all Crabs. 



The genera Sesarma and Cyclograpsus, belonging, like the 

 preceding, to the family Grapsidre, contain species living in 

 holes on the shore. These species possess the same posterior 

 respiratory fissure ; but it is difficult to see this gaping, as the 

 animals rarely open it, indeed only when they have been a very 

 long time out of the water. This is due to a very curious 

 arrangement, which does not exist in the preceding species, and 

 which enables these animals for a long time to respire the air 

 dissolved in the water that bathes their branchiae. The ptery- 

 gostomian region which separates the apertures for the ingestion 

 and egestion of water is, as it were, reticulated, and bristles 

 with small recurved hairs, already indicated by Milne-Edwards. 

 The water issuing from the egestive orifice spreads in an instant 

 over this network of hairs, and becomes saturated with air, after 

 which it is conducted by a special arrangement into the ingestive 

 aperture. The same portion of water may thus pass through 



