Oct., i894. TEMPERATURE AND MARINE ANIMALS. 277 



different temperature in the different seas and different latitudes. 

 Moreover, if temperature is the determining factor, Polar animals 

 ought to be the same as abyssal ones in lower latitudes, which is 

 denied by Heilprin. 



In the whole of this argument no clear distinction has been made 

 between three classes of facts and relations : — 



1. In the first place, no distinction has been made between 

 pelagic animals, the Plankton and the Benthos, i.e., the life confined 

 to the shore or ground, animals either sessile or capable only of 

 creeping, in contrast to the Nekton, the freely-swimming animals, 

 capable of considerable movements, able to stem the tide and migrate 

 at will. 



2. Secondly, without further consideration a parallel has been 

 drawn between the horizontal and the vertical distribution with 

 reference to temperature, while the latter becomes complicated by 

 other factors, too, as pressure, want of light, etc. Moreover, the 

 gradations in the change of temperature in different depths are not 

 all proportional to the amount of the depths themselves. 



3. In the above argument no distinction has been made between 

 eurythermal and stenothermal animals. 



A few words ought to be said to explain these terms (first 

 adopted by Moebius). We might give a definition by saying that 

 eurythermal animals can bear great differences of temperature, steno- 

 thermal animals cannot, and this can be represented in a graphic 

 manner. Each animal has a certain optimum of temperature, at 

 which it flourishes best. A number of degrees above it its meta- 

 bolism cannot go on ; and in a similar manner, at a certain degree 

 below this optimum, the metabolism ceases also. Thus we obtain 

 for each animal three points in the thermometrical scale — optimum, 

 maximum, and minimum. Now in some animals the space on the 

 thermometrical scale between maximum and minimum can be very 

 great : these are eurythermal animals. In other cases it is small : 

 these are the stenothermal animals.' 



It is clear that eurythermal animals cannot be made use of 

 either as a proof or a contradiction of the influence of temperature. 

 Many animals of the shore, especially molluscs and echinoderms, are 

 proved by experiments to be eurythermal. If a bivalve, for instance, 

 is found both in the North Polar Sea and in the lower strata of the 

 German or English Sea, nothing is proved for the influence of 

 temperature — ^just as little as the occurrence of a eurythermal animal 

 at the surface both in the Mediterranean and at Spitzbergen would 

 be a proof against it. This has often been overlooked. 



Moreover, it is easy to understand that it is necessary that the 



1 It is obvious that warm blooded animals can bear greater differences of 

 temperature because they have their mechanism of heat regulation in their own 

 body ; they are therefore eurythermal. The poikilothermal animals, on the contrary, 

 to which] the animals of the sea belong almost exclusively, are more stenothermal 

 a priori. 



