306 Prof, Dr. G. Pfeffer on the Mutual Relations 
cooling of the climate, died, or migrated towards the equator ; 
while those that preferred cooler water, and had till then 
inhabited the subsurface-water, or, at all events, had not 
found their optimum of temperature in the surface-water, 
were now able to distribute themselves unrestrictedly over 
the whole surface and subsurface-water of their former 
habitat. There is no ground for the theory that the simi- 
Jarity of the faunas of higher latitudes depends on adapta- 
tion; the genera remained unchanged before and after the 
separation of the faunas, as is proved by the comparison of 
the successive faunas of ‘Tertiary and recent times. 
(2) We have now to consider the question whether science 
requires us to believe that, in the times when a climate of 
tropical heat prevailed in our latitudes, the equatorial regions 
must have possessed a hypertropical climate, which would 
make life impossible. 
In the first place, we have no ground for assuming that 
the present-day temperature is the highest degree of warmth 
that tropical animals are capable of enduring, or even that it 
affords their optimum of warmth. On the contrary, we have 
observations enough to show that tropical animals can very 
well endure a teniperature considerably higher than that of the 
tropical surface-water. We know, too, that along the conti- 
nental west coasts the cold currents extend into the tropical 
zones, and that, within these, cold deep water wells up, and 
the warmth of the suface-water is thereby materially 
lessened. As the causes of these horizontal and vertical 
water-movements are not local but telluric, they have held 
good for all ages. We can therefore imagine that, at a 
time when the surface-water on the east coasts was actually 
uninhabitable by living beings on account of its great heat, 
there may have been, in the regions of the continental west 
coasts, a climate which animals with the same warmth- 
requirements as our present-day tropical animals could quite 
well endure. 
It has also been shown that it is in no way proved, as 
many have assumed on a prior? grounds, that the same 
difference of temperature must have existed between the 
temperate latitudes and the equinoctial zones in the Karly 
‘Tertiary or the Later Cretaceous period as obtains at the 
present day. ‘This question has been discussed by me from 
the climatological point of view and by Dubois from the cosmo- 
logical. ‘These discussions do not, however, lie within the 
scope of our present problem. 
