On the Variations of certain Crustacea. 329 



true, we ouglit in such and such a case to find such and such 

 peculiarities." He then set to work to investigate on this 

 basis, and the following is a very striking example of his 

 results. 



Crabs are divided into several natural families, so grouped 

 because all the species of each family [possess certain cha- 

 racters in common which distinguish them from those of neigh- 

 bouring families. This, according to Darwin's hypothesis, is 

 the natural result of their descent from one common ancestor. 

 Then the species of all these families, which we may call 

 A, B, and 0, present certain ordinal characters common to all, 

 and due to the fact that the families A, B, and 0, descended 

 from a more remote type X. This may be more plainly shown 

 as follows : — 



X 



a a' a" a'" — a 1 b b' b" b'" — b 1 c c' c" c"' — c L 

 First family. Second family. Third family. 



Now it is^remarkable that in each of these families we find, 

 as exceptions to the normal mode of life of the crabs, certain 

 terrestrial species. It is permissible to suppose, a priori, 

 that these must present certain modifications of the respiratory 

 apparatus, enabling them to breathe air, and if each terrestrial 

 species has gradually renounced, for itself, the aquatic mode of 

 life, there is every probability that each would present a modi- 

 fication, sui generis, very different from those presented by 

 others. If, on the contrary, observation proved that all these 

 terrestrial species present the same modification' of the respi- 

 ratory apparatus, the Darwinian theory could only accouut for 

 them by assuming that these terrestrial species belonging to 

 various families, which we may distinguish as a 1 , b 1 , c 1 , descended 

 from a common type, 0, which had already acquired the organic 

 conditions of aerial respiration. But then the theory would 

 contradict itself; for while the study of the respiratory organs 

 would compel us to make a 1 , b l , c 1 , descend from 0, the dis- 

 tinctive characters of the families to which they belong lead 

 us to assign to them a different origin, namely, from X through 

 A, B, and 0. 



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 to Dr. Mullei\ If he found in 

 the terrestrial species of different families the same arrange- 

 ment 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, 



