EFFECTS OF CHANGES IN THE CHEMICAL NA TURE OF THE 'I\ lEDIUM. Every one is aware how rapidly most sea animals, all except the migra tory fishes, such as salmon, die when placed in fresh water. The effect of living by the sea on land plants is to thicken their leaves. The pali sade cells are more munerous and larger than in leaves of the same plants when grown inland. Apparently, the sea-salt is the cause of this al teration, as plants cultivated in an artificially salted soil yield thicker leaves. Such a change as this is the result of the direct action of a changed environment.
The effect of a change from fresh to salt water on delicate forms, like the phyllopod crustaceans (Branchipus, etc..), is to dwarf them. Thus, the little brine-shrimp (q.v.) is apparently a dwarfed and otherwise modified form of some fresh-water ancestor.
Experiments have shown that the brine-shrimp varies greatly with differences in the density of the water in which it lives, with the result that here we have a transformation of one species into another. One form living in strong brine
has a longer abdomen than others living in a weaker solution. This has been observed in nature, and also in forms living in the Inborn The lakes near the Sea of Aral are known to vary in degree of saltness at different seasons of the year: the result of this change from saline to comparatively fresh water causes marked varia tion in the pond-snails. so' much so that the extreme varieties might be regarded as distinct species. So with the eockle-sbells: the semi-fossil ones on the border of lakes which once formed a part of the Aral Sea vary greatly.