An interesting case is that of _Daphnia rectirostris_, a small crustacean living sometimes in fresh water, at other times in water containing salt and also in salt lakes. There are two forms, corresponding to the conditions under which they live, and it is said that the differences are of a kind that suffice to separate species from each other. In another crustacean, _Branchipus ferox_, the form differs in a number of points, according to whether it lives in salt or in fresh water. Schmankewitsch says that, had he not found all transitional forms, and observed the transformation in cultures, he would have regarded the two forms as separate species. The oft-quoted case of Artemia furnishes a very striking example of the influence of the environment. _Artemia salina_ lives in water whose concentration varies between 5 and 12 degrees of saltness. When the amount of salt is increased to 12 degrees, the animal shows certain characteristics like those of _Artemia milhausenii_, which may live in water having 24 to 25 degrees of saltness. The form _A. salina_ may be further completely changed into that of _A. milhausenii_ by increasing the amount of salt to the latter amount. Among domesticated animals and plants—a few instances of which have been already referred to—we find a large number of cases in which a change in the environment produces definite changes in the organism. Darwin has made a most valuable collection of facts of this kind in his “Animals and Plants under Domestication.” He believes that domesticated forms are much more variable than wild ones, and that this is due, in part, to their being protected from competition, and to their having been removed from their natural conditions and even from their native country. “In conformity with this, all our domesticated productions without exception vary far more than natural species. The hive-bee, which feeds itself, and follows in most respects its natural habits of life, is the least variable of all domesticated animals.... Hardly a single plant can be named, which has long been cultivated and propagated by seed, that is not highly variable.” “Bud-variation ... shows us that variability may be quite independent of seminal reproduction, and likewise of reversion to long-lost ancestral characters. No one will maintain that the sudden appearance of a moss-rose on a Provence rose is a return to a former state, ... nor can the appearance of nectarines on peach trees be accounted for on the principle of reversion.” It is said that bud-variations are also much more frequent on cultivated than on wild plants. Darwin adds: “These general considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability.”