CONDITIONS OF EXISTENCE 13 



are represented in the ocean, whereas important groups are wholly 

 absent on land and in fresh water, and no animal phylum is restricted 

 to these environments, unless the very small fresh-water group Gas- 

 trotricha is considered a phylum. The circumstance that only marine 

 organisms are known from the older fossiliferous geologic strata also 

 speaks for the marine origin of life.* 



Salinity.— The extent of the variation in salinity which can be 

 borne is very different for different animals. Some are influenced by 

 slight changes in salt content, such as reef corals and amphibians; 

 others withstand much variation in this respect, as, for example, the 

 annelid, Nereis diversicolor, and among fishes, the salmon and stickle- 

 back. The amphipod, Niphargus, occurs in water with 8.7 per cent 

 salt in solution, while, as is well known, the brine shrimps of the genus 

 Arternia tolerate great changes in salinity. 7 The first-named type of 

 animals may be designated as stenohaline, the latter as euryhaline. 



Humidity. — Terrestrial animals cannot dispense entirely with 

 water, but some occur only under very humid conditions whereas 

 others are partial to dry situations. Animals tied to narrow limits of 

 variation in atmospheric humidity may be referred to as stenohygric; 

 those which can withstand great variations of humidity are euryhygric. 

 Worms, most snails, amphibians, and water buffaloes are examples of 

 stenohygric animals in moist situations; the camel is a stenohygric 

 animal of arid country. Euryhygric animals are numerous in many 

 groups, especially among the insects, birds, and mammals. 



Temperature. — The temperature limits between which animal ac- 

 tivity is possible are not very wide. The lower limit is necessarily 

 defined by the freezing temperature of the body fluids, a few degrees 

 below that of pure water. The upper limit of temperature for most 

 activities of living matter lies between 40° C. and 50° C, at which 

 point the dissolved albuminoids probably undergo destructive chemical 

 change. Leathes and Raper 8 have recently suggested that temperature 

 toleration is a matter of the relation of fats rather than of proteins; 

 the more liquid fats occur in animals adapted to lower, the more 

 solid, in those found in higher, temperatures. The effects of the two 

 temperature limits are widely different. The suspension of the activity 

 of protoplasm produced by high temperatures usually causes the death 

 of the animal; the suspension of activity as a result of low tempera- 

 tures, on the other hand, is not necessarily fatal, and active life may 

 be resumed with the return of a favorable temperature. Many ani- 

 mals are accordingly able to exist in regions where they are forced 



* The opposite theory is maintained by some authors* 



