306 Evolution and Adaptation 



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 milhauscnii, 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 domesti- 

 cated 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 reproduc- 

 tion, and likewise of reversion to long-lost ancestral charac- 

 ters. No one will maintain that the sudden appearance of 



