1048 PRINCIPLES OF STRATIGRAPHY 



temperature is reached ; this, where not affected by oceanic currents, 

 varying mainly with the change in latitude. It is to this region 

 of unvarying temperature that many of our littoral vagrant benthos 

 descend on the approach of winter; so that, as every collector 

 knows, many tide pools and sandy bottoms of moderate depth be- 

 come deserted during the colder months. Many marine organisms 

 are eurythermal, i. e., able to bear a considerable range of tempera- 

 ture. Others, again, are stenothermal, a comparatively slight devia- 

 tion from the normal temperature of their medium being fatal to 

 them. The larvae or eggs of stenothermal animals are often able 

 to resist very great changes of temperature which would destroy 

 the adult animal. Thus the winter eggs of some of the lower Crus- 

 tacea, the germs of Bryozoa and of fresh water sponges, resist any 

 degree of cold, while the full-grown individuals perish in the au- 

 tumn. Many insects cannot survive the winter, though the eggs 

 and the embryo within the eggs commonly withstand the severest 

 cold. On the other hand, the young may be more susceptible to 

 changes in temperature than the adult. Brooks found that, in the 

 case of the oyster, the difference of 2° or 3° F. in the temperature 

 of the water was sufficient to kill the whole larval brood. Thus, 

 as Dall says, "By inhibiting natural increase ... a species 

 may be as sharply limited in its permanent range as if material 

 barriers interposed." (Da\\-g:2i8.) 



Cold-blooded animals can usually withstand a lowering of tem- 

 perature better than warm-blooded animals. Thus frogs and toads 

 can be frozen into the ice and survive, and so can certain fishes 

 {e. g., Salmon, etc.). 



In all cases a rapid change of temperature is more fatal to or- 

 ganisms than is a gradual one ; for many normally stenothermal 

 types can by degrees adapt themselves to a higher or lower tem- 

 perature. When once acclimatized it is the change fron>the normal 

 temperature of the new habitat which affects the organism. Thus 

 members of a species acclimatized to more tropical regions will be 

 affected by a fall of temperature to a point where members of the 

 same species in more northern regions are wholly unaffected. 



Currents. One of the marked features of the media characteriz- 

 ing the three organic realms is the frequent presence of currents 

 within them. These may be temporary or fixed, and, according to 

 this characteristic and their strength, they become powerful factors 

 in the influence which they exert upon the distribution of organisms 

 in the realms in question. In the air, currents are most numerous 

 and also more variable, though certain air currents, as the "trade 

 winds," have a great constancy within certain limits. Irregular 



