282 CAUSES OF VARIATION 



SECTION IX EFFECT OF SALINE SOLUTION UPON 

 DEVELOPMENT IN AQUATIC ANIMALS 



Sea water differs from fresh water in two particulars, salinity 

 and density, both of which exert marked influence upon animal 

 life and between which it is often difficult to discriminate. A 

 goldfish plunged into sea water at first shows " violent incoor- 

 dinated movements," but shortly " becomes immobile and rises 

 to the surface by virtue of its lower specific gravity." l 



" The effect of fresh water upon marine organisms is equally 

 striking. They go immediately to the bottom and move with 

 difficulty. Swimming animals swim badly if at all, and small 

 fishes have to make much exertion to rise to the surface." x 



On many marine animals, as mollusks and fish, fresh water 

 acts as an anaesthetic, the mollusks soon yielding to paralysis, 

 the fish appearing to suffer from lack of air. " The respiratory 

 movements become deep and rapid. ... The tissues become 

 swollen so that soft-bodied animals are visibly deformed, in 

 fishes the eyes are forced out, the foot of gastropods swells, the 

 blood corpuscles swell up and burst, and muscular tissue may 

 increase as much as six times in volume." 2 



Many of these effects are clearly due to differences in pres- 

 sure which may amount to many atmospheres, but it remains 

 to separate, so far as may be, the effects of salinity from those 

 of specific gravity. 



Rather startling claims have been made from time to time 

 as to the conversion of one species into another by altering the 

 degree of salinity. Further investigation seems to show that in 

 all such cases intermediate forms are known to occur, which 

 argues that the two forms which had been recognized as dis- 

 tinct were not both good species ; that is to say, it is a case of 

 one species with wide variability as to certain characters, not that 

 of two distinct and well-defined species. If, however, differences 

 in salinity are effective in bringing about alterations in even a 

 single character, the fact is of interest here, no matter what 

 specific lines should be drawn by the biologist. 



1 C. B. Davenport, Experimental Morphology, Part I, p. 79. 

 - Ibid. pp. 79, 80. 



