as supported by Observations on Crustacea. 41 1 



any group of animals is at the same time the genealogical tree 

 of that group. Dr. Fritz Miiller has endeavoured, in the first 

 place, to construct this natural classification or genealogical tree 

 for the class of Crustacea; and having constructed it, he has 

 deduced from its structure certain necessary consequences. 

 These deductions he has then endeavoured to verify. If they 

 could not be verified, this would be a fatal blow to the Darwinian 

 theory ; but if they proved true, they would furnish, if not a 

 proof, at least a strong presumption in favour of the theory. 

 Hitherto his deductions have been verified ; and thus his work 

 presents us with a remarkable example of important results in 

 natural history obtained by a purely deductive method, in oppo- 

 sition to most of the discoveries in that science, which are made 

 by means of a sort of inductive groping. 



Zoologists distinguish several natural families of Crabs. The 

 species of one of these families, which may be designated as 

 a, a', a", a'" } &c, have certain characters in common ; and this 

 is the case, according to Darwin's hypothesis, because they 

 descend from a common ancestor, A, which already presented 

 these characters. In the same way, the species b, b', b", b"', 

 belonging to a second family, present all the characters of the 

 family because they descend from a common ancestor, B ; and 

 the species c, c', c", c" of a third family have certain common 

 characters derived from an ancestor C, and so on. Lastly, the 

 species of all these families present certain ordinal characters 

 common to all, and due to the fact that the forms A, B, C 

 descended from a single primitive type, X. Thus the genealogical 

 tree of these Crustacea would be as follows : — 



X. 



B. C. 



a, a' } a", a'". ... a n 1), b', b", b'". . . . ? c, c', c", c»> . ...c n 

 1st family. 2nd family. 3rd family. 



Now it is to be remarked 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 respire air. And it is possible to 

 imagine a multitude of arrangements capable of leading to this 

 result; and if each terrestrial species has gradually renounced 

 the aquatic mode of life on its own account, there is every pro- 

 bability that each of them would present a modification sui 

 generis, very different from those presented by the others. If, 



27* 



