No. 410.] THE EUROPEAN FAUNA. II3 
of dispersal to a minimum because the many happy coinci- 
dences necessary for a species to establish itself on a distant 
shore must be exceedingly rare. But granting this, we must 
not forget that while the recorded actual observations of 
direct accidental transplantation and subsequent establishment 
are few and far between, our entire experience covers scarcely 
a couple of centuries, while the periods of which we here treat 
are reckoned in thousands, possibly tens of thousands of years. 
And how many lucky chances may not have presented them- 
selves in ten thousand years! Moreover, the distances with 
which we have to do in the present case only look formidable 
in the Mercator projection, while in reality they probably did 
‘not exceed 150 miles. I believe it to be a distinct mistake to 
suppose that all classes and kinds of animals and plants have 
followed the same lines of distribution in time and space. As 
a matter of fact, it may be even said that there are scarcely 
two species which have exactly the same history. Each one 
must be worked out separately, and too sweeping generaliza- 
tions are dangerous at the present state of our knowledge. 
It is one of the distinct merits of a book like Dr. Scharff's 
that it makes plain these defects in our knowledge. It is a 
kind of stock-taking by which we find out just how our busi- 
ness stands. It must then be admitted with regret that the 
status is not as satisfactory as one might have reason to expect. — 
There is yet a great uncertainty as to the exact and detailed 
distribution of many of the larger and more important animals 
in the Arctic regions and in Europe. The grosser facts are 
known of course in a general way, but they are not sufficient 
for the purpose. The finer details are still unknown, or if 
known in some isolated cases are unavailing because they are 
as yet only isolated. American mammalogists, for instance, 
have studied some of the species here treated of in so far as 
they relate to American forms, but this knowledge is at pres- 
ent a dead one, because the corresponding Asiatic and Euro- 
pean forms are still in chaos, or vice versa. Dr. Scharff's book 
shows that there is still much necessary work to be done by 
the “splitter” of species and subspecies, but it also shows 
that this work must be done with some purpose in view and 
