278 NATURAL SCIENCE. Oct., 



animals of the Nekton, i.e., the animals which swim well, should not 

 be too sensible to temperature ; for by their power of moving either in 

 a horizontal or in a vertical direction, they pass easily into very 

 different conditions and have to be adapted to them, if the faculty of 

 swimming is to be of any use to them at all. As a matter of fact, we 

 find that the large swimming animals, such as crustaceans and fishes, 

 are, in general, eurythermal. 



The geographical distribution of whales seems at first to contradict 

 this consideration, but in reality it does not. For if we find some of 

 their species chiefly inhabiting the high north or south latitudes, it is 

 not because they are unable to endure higher temperatures, but because 

 only in those regions can they find the enormous quantity of organic 

 substance, of Plankton, which they require for their gigantic 

 bodies. 



Here we touch another point of importance. In high north and 

 south latitudes, the production of Plankton, the primordial food, the 

 algs, the small Crustacea, etc., is extraordinarily great, because the 

 variations of temperature are here small, and therefore the life- 

 conditions are very favourable, in spite of a low degree of temperature. 

 This also has contributed to obscure the question of the influence of 

 temperature. If an animal cannot exist below or above a certain 

 degree of temperature, it is clear that the determination of the average 

 temperature of a place is of small zoogeographical value ; for this 

 average temperature can just as well be the mean between two 

 temperatures suitable for the animal as between two which are 

 harmful or fatal to it. The attempt has therefore been made to draw 

 lines of greatest cold as decisive for the question, and these are termed 

 isocrymal lines by Dana. Not much, however, is gained by that ; 

 first, because we ought to regard the lines of greatest heat as well, 

 and secondly, because it is not the degree of temperature alone, but 

 the extent of variation which has a harmful influence. It is proved, 

 in some instances, that the same animals can live at very different 

 temperatures, at 3°, 7*^, 15° Centigr., if only this temperature at the 

 respective localities remains fairly constant to 3'^, 7°, 15° Centigr. 



These are eurythermal animals, but they are, nevertheless, stenoi 

 (restricted), not with regard to temperature, but with regard to its 

 variations. 



I think a strange occurrence in marine life, upon which Semper 

 laid stress, can be explained in this manner. Some Hexactinellids, 

 typical sponges of the deep sea, occur in the Pacific Ocean, in very 

 different temperatures ; even at a comparatively small depth in 

 tropical parts, in which the temperature is about 10^ higher than in 

 greater depths of the China Sea, in which they are to be found also. 

 The factor which all these localities have in common, besides the 

 absence of currents, is slight variation of temperature. 



We have seen that the Nekton animals, being eurythermal, 

 cannot be appealed to in pyoof of anything regarding the influence of 



