“The question ... resolves itself into this: which probability or hypothesis do _the facts_ of the case seem to favour most, viz. that _indefinite_ variations arise from some assumed internal causes, of which variations only those in harmony with the environment survive, and are said, therefore, metaphorically, to be selected by it; or is it that the external forces of the environment excite the variability which is inherent in plants, and call into action the responsive power of the protoplasm in the various species of plants, which thus _all_ tend to put on the same, or similar, or at least adaptive and _definite_ variations of one sort or another, so that there are _no_ indiscriminate or wasted variations[92] at all? I know an abundance of facts which support the latter contention, but none whatever in illustration of the former hypothesis.” Here is the action of the dominants in evolution placed in the clearest light. To prove the truth of Professor Henslow’s contention it is necessary not only to study organisms in situations where they have been established for many generations or centuries, but to see how they behave on transportation to a new kind of environment. The cases which can be adduced are numerous and convincing. Thus Mr. D. Dewar reported to Mr. Henslow that on introducing at Kew a cress, _Arabis anachortica_, found in cave-like situations in the Alps, and having very thin, papery leaves, it turned, when raised from seed, into a different species, _Arabis alpina_. The change took only three generations to accomplish.[93] Bulbous roots have it among their functions to store up moisture for the plant they belong to. Haeckel has shown that the grass _Poa bulbosa_, on being cultivated in moist soil, almost lost its bulbous character. Contrariwise we find that many plants not bulbous elsewhere are observed to be so when growing on the dry Karoo in South Africa.[94] Spines on a plant are usual accompaniments of dryness in soil or atmosphere. _Ononis spinosa_ has an excessively spiny variety, termed _horrida_, which is found on maritime sands. Grown in very rich moist situations, it gradually loses its spines and they ultimately disappear entirely.[95] In the animal world experimental cultivation is not at all so easy, but the facts observed all go to support the view that the response to environment is direct and definite. The small shrimp-like crustacean, _Artemia salina_ is a case frequently quoted. It lives in salt pools by the Black Sea, and it has been found that by breeding it in water of which the salinity is gradually decreased, the creature in a few generations assumes a type commonly assigned not merely to a different species but to a different genus—_Branchipus stagnalis_.[96]