ELEMENTARY VITAL PHENOMENA 183 



Since the conditions upon the earth's surface have slowly and con- 

 stantly changed from the time of the incandescent nebula down to 

 the present, since fairly rapid changes of the external conditions of 

 life continually appear in locally restricted regions, and, finally, since 

 all organisms are constructed even to the smallest minutiae in a 

 manner corresponding perfectly to both general and special con- 

 ditions, organisms must become adapted to their external conditions 

 constantly and in proportion as the conditions themselves change. 

 If this ratio between the change of external conditions and the 

 change of the form of organisms had not existed in the past, there 

 would have appeared within a conceivable time an extraordinary 

 lack of fitness in the structure of organisms. But the cases in 

 which an organ seems to be superfluous are relatively rare, and 

 injurious mechanisms perhaps do not exist at all. 



The mode of adaptation of organisms is a double one : an indi- 

 vidual, or personal, and a phyletic, or racial, adaptation may be 

 distinguished. The two occur very differently. 



Individual adaptation acts only within very narrow limits, and 

 in the phylogenetic changes of form has, perhaps, only a subordinate 

 importance ; it has, indeed, no importance whatever in phylogeny, 

 if the inheritance of acquired characteristics does not take place, 

 for it consists in the fact that changes in the external environ- 

 ment cause direct changes in the organism itself according to the 

 different factors of the environment. Individual adaptation usually 

 expresses itself much more clearly in habits, manner of life, etc., 

 than in form. A man, put under other conditions than his 

 customary ones, in another land and among other people, adapts 

 himself to his surroundings gradually in the course of years, and 

 gradually adopts the customs, usages, activities and mode of life 

 of the new people. Much more seldom is there observed in 

 organisms a change in body-form through individual adaptation 

 to vital conditions, especially because much more profound changes 

 in the conditions are necessary to cause it, and these are not so 

 easily endured as the relatively slight changes that lead to adapta- 

 tion in manner of life. A relatively slight change in the compo- 

 sition of the water in which aquatic animals live, leads in most 

 cases to death. Marine animals placed in fresh water and fresh- 

 water animals placed in sea water usually die ; only a few forms 

 have adapted themselves to both, especially such as live at the 

 mouths of rivers, like certain fishes. A crustacean, Artemia 

 salina, is very interesting in this connection. Schmankewitsch 

 (77) established the very interesting fact that this small 

 animal living in salt water can change itself, by slowly becoming 

 accustomed to a higher or lower percentage of salt, into a dif- 

 ferent form of crustacean in water of greater concentration into 

 Artemia Milhausenii, in fresh water into Branchipus stagnalis, two 

 forms having wholly different characteristics (Fig. 65). Similar 



