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 tlieir optimum of temperature in the sui face-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- 

 larity 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 higiiest degree of warmth 

 that tiopical 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 temperature 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 surface-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 priori grounds, that the same 

 difference of temperature must have existed between the 

 temperate latitudes and the equinoctial zones in the Early 

 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. 



