DISTRIBUTIONAL DIFFICULTIES. 333 
the rivers of New Zealand, Australia, Madagascar, and 
South America, and become fresh water Parastacide, is 
an assumption which is justified by the analogy -of the 
fresh-water prawns. It remains to be seen whether 
marine Parastacide 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 etiology of the 
crayfish will some day be establislied, 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 
