VARIATION ( Lat. rariatio, from rariare, to change, vary). Structural or functional de viations from the parent form or type. Varia tions, which are apparent to all, are attracting the careful attention of working naturalists, whose tendencies are to cease theorizing and to bend all their energies toward the discovery and elassilication of facts and of the causes producing variation. Species are unNitally variable; there are plastic as well as rigid. ones. Nature pre vents over-variation by competition; otherwise crowding of species would result. The greatest range of variation occurs in cultivated plants and domestic animals; yet the turkey dues not vary, whether living in America or Europe, nor does the guinea-fowl in America depart from the African type: and the eat varies much less than the dog. Wide-ranging, inuch diffused, and com mon species vary most, and those of large genera more frequently vary than those of genera con taining but one or only a few species. Persistent or ancient and cosmopolitan forms have not varied all through the immense periods of their history.
There has been much discussion as to whether variation is fortuitous or definite. It was claimed by Darwin that variation is by clianee, though he adds that this is 'a wholly invorrect expression,' for in nature there is no such thing as chance, the expression serving "to acknowledge plainly our ignorance of the cause of each par ticular variation;" and he states that "the direct tu•tiou of changed conditions leads to definite or indefinite results." But it is more probable that, though variations may in some cases seem to be fortuitous, in general throughout geological variation has been regulated by changes in the physical and biological conditions of the environ ment along certain determinate lines, the varia tions becoming, permanent when they prove useful.
That the direction of variation is in accordance with and dependent on the surroundings is shown by the atrophy of eyes in animals living in dark ness, the reduction in the visual organs varying in degrees dependent on the length of time and on the number of generations the animals have lived in darkness. The reduction in the number of limbs and toes is conditioned by the mode of life. The variation in the colors of animals is in direct relation with the varying amount of light and shade to which they are exposed. See Pro TECTIVE COLORATION.
The causes of variation are in the main ob scure, but primarily variation is clue to the changes of the environment, i.e. to the differences in atmospheric pressure, electricity, gravity, light, to changes of temperature (heat and cold), and of the mechanical and chemical state of the water or air, as well as the kind and amount of food. The causes are mainly external, but some are apparently internal, such as reproduction. These physical agents have been called the primary factors of organic evolution (q.v.) operating on the most primitive and plastic one celled forms of life, which in the beginning led to the origin of the different types of life or lines of development.