of the Arctic and the Antarctic Faunas. 321 
is in the tropical zone a considerable discontinuity in the 
distribution-region of several hundreds of genera of surface- 
water animals occurring in the higher northern and southern 
latitudes, and also in the distribution of very many—in any 
case far more than a hundred—species of higher latitudes. 
Hither the discontinuity is really present in the tropical zone, 
or it appears to be so because of the present incomplete state 
of our knowledge. ‘The reasons for the first supposition we 
have already discussed. Reasons in favour of the second 
case undoubtedly exist also, and I believe that, with the 
further progress of our knowledge, some regions of distri- 
bution which are now believed to be discontinuous will be 
found to be continuous. But that this may prove so in all 
cases 1S an assumption which receives no justification either 
from general considerations of probability or from the present 
state of our knowledge. And it is not only our right, but 
our duty to base our theoretical assumptions on the state of 
knowledge at the time. But even if we go the length of 
assuming that all bipolar genera and species may disappear 
from science, will that affect a single point in our theories of 
the relation of the arctic and antarctic faunas? The Karly 
Tertiary roots of our present fauna remain the same whether 
bipolarity exists or not; and the similarity between the 
faunas of higher latitudes also remains the same, whether the 
distribution-regions of the genera or species be continuous or 
not, 
The possibility, however, that a continuous distribution 
through the deeper waters of the tropics may be demonstrated 
for all the coeval surface-water inhabitants of the higher 
northern and southern latitudes is simply not to be thought 
of. The marine plant-world of the higher latitudes exhibits 
quite a pronounced bipolarity ; and the idea that this should 
extend through the lightless layers of the tropical subsurface- 
water beyond the equator cannot be entertained. ‘The same, 
of course, holds true of all animals which are directly depen- 
dent on the plant-world. So that, as a general principle of 
explanation, we may dismiss the hypothesis that the surface- 
water animals of higher latitudes have a continuous distri- 
bution through the deeper layers of the subsurface-water of 
the tropics. ‘his, of course, does not prevent us from sup- 
posing that the assumed connexion, which must have existed 
at one time, does actually occur in some cases at the present 
day, both in the deeper strata and in the surface-water. 
There is still another possible way in which the examples 
of bipolar genera and species, which have hitherto been 
present in literature, may disappear from science ; that is, by 
