DISTRIBUTIONAL DIFFICULTIES. 333 



the rivers of New Zealand, Australia, Madagascar, and 

 South America, and become fresh water Paras tacidce, is 

 an assumption which is justified by the analogy of the 

 fresh-water prawns. It remains to be seen whether 

 marine Parastacidce still remain in the South Pacific 

 and Atlantic Oceans, or whether they have become 

 extinct. 



In speculating upon the causes of an effect which is" 

 the product of several co-operating factors, the nature 

 of each of which has to be divined by reasoning back- 

 wards from its effects, the probability of falling into 

 error is very great. And this probability is enhanced 

 when, as in the present case, the effect in question 

 consists of a multitude of phenomena of structure and 

 distribution about which much is yet imperfectly known. 

 Hence the preceding discussion must rather be regarded 

 as an illustration of the sort of argumentation by which 

 a completely satisfactory theory of the aetiology of the 

 crayfish will some day be established, than as sufficing 

 to construct such a theory. It must be admitted that 

 it does not account for the whole of the positive facts 

 which have been ascertained ; and that it requires sup- 

 plementing, in order to furnish even a plausible explana- 

 tion of various negative facts. 



The positive fact which presents a difficulty is the 

 closer resemblance between the Amur-Japanese crayfish 

 and the East American Cambari, than between the 



