EVOLUTION. 323 EVOLUTION. each other in color, size, etc., i. e. in characters l,.,s pronounced than those of species. Darwin calls a variety an incipient species, or a species in process of formation. Local varieties are fixed variations of species with a wide range; they are restricted to small areas shut in try mountain ranges, etc. They are thus due to dif- [ e ren1 local environments, to differences in tem- perature, altitude, dryness or moisture, soil, etc. Local varieties, says Wallace, are the tir-t steps in the transition from varieties to species. arie- ties are subdivided into 'races,' though the two terms arc often used interchangeably. Huxley ealls a rac< 'a propagated variety.' Races are subdivided Qto 'breeds,' and these into 'strains' — the numerous breeds of pigeons, as the fantail, pouter, barb, tumbler, English carrier, etc., are examples of breeds, while a strain is the bast recognizable variation from some racial i iikiiiu d eslie horses, wc speak of the Arabian, Clyde, or Percheron breed; of the Morgan or Hambletonian strains. The American trotting horse is a breed which has been formed bj arti- ficial selection since the year 1822. Spoets. Much interest is now felt in the mat- ter of 'sports' or aberrations. A sport may be debited as an individual which is born of ordi- nary parentage, but differs in some marked physical or menial characteristic. Sports in cul- tivated plants or domestic animals appear with- out any known cause, whereas aberrations in moths and butterflies are readily produced by artificial changes in temperature, humidity, or dryness. Among animals, the famous A neon or otter sheep is as remarkable as any. By careful breeding with normal sheep, this long-bodied, crooked-legged sheep became the progenitor of a flock resembling the first aberration. Nature abhors aberrations, and by natural selection wipes them out. Yet Galton thinks that the transmutation of species is due to sports, which, if useful, are seized upon, and perpetuated by natural selection. With this view we sympa- thize, for the occurrence, claimed by Darwin, of vast numbers of intermediate forms between what are now 'valid' species, is a hard doctrine. Sports may be divided (1) into 'teratoid' sports, the result of discontinuous variation, in fact monstrosities, and (2) 'biotic' sports, which are due to more or less sudden changes of the conditions of life. The remarkable experiments of Standfuss show that, in lepidoptera the crossing of an aberration with the parent form may often result not in the production of intermediate types, but in the sharp cleavage of offspring into two groups, each resembling one parent and not the other. Factors of Organic Evolution. We will now pass on to the mechanism of evolution, the means by which species and genera, and even orders, classes, and phyla, have been evolved. These factors are ( 1 ) changes in the environ- ment, and (2) dynamic or physiological, i.e. the result of movement, strains and stresses, due to the exercise of organs. Effects of Changes in the Cosmical Envi- ronment of the Genesis of Life-Forms. When the earth was cooling down from a gaseous to a solid condition, it is most probable that the changes in the cosmical environment had an im- mediate and creative or modifying effect on the beginnings of life. It is certain that the action of the same cosmical agents, such as motion, gravil i. iiiy, etc., which have determined i hi phi rical shape oi the planet -. as well as of a drop of water, has been concerned in determining i he shapes oi cells, of eggs, oi the simpb isms, and is the basis of all physiological phe- nomena, as well as of motion in animals. Plant and animal life are influenced in a way we do not understand by electricity; they arc al-o influenced by variations in the pressure of the air. Jaegei i la ima I hal 1 be force oi graA itation i- t he primitive morphogenetic factor in the develop- ment of animals. In the growth of plants the influence of gravitj and ligh! is marked. The influence of gravity on the form of shells is no- ticeable. To this has been, by Hyatt, attributed the asymmetry of univalve shells. The mechanical state oi the medium is impor- tant in modifying the shape of animals. The spindle-shaped body of fishes enables them to cleave the water; so it, is with the shapes of winged animals, whether insects, birds, or ptero- dactyls. When s e terrestrial mammal driven by necessity or competition with its fel- lows to seek a livelihood in the sea, and thus gave origin to the order of whales, its body un- derwent a transformation; it became fish-like in shape, and while from disuse it lost its hind limbs, the fore legs were converted into fins. When animals of very different type-, such as the earthworm, many sea worms, multitude- of parasitic worms, the boring larva? of insects, live in a denser medium than water, and have been obliged to burrow in the soil, or in the dense tis- sues of their hosts, the body tends to become elongated, cylindrical, and pointed at each end. Effects of Changes in the Chemical Na- ture of the Medium. Every one is aware how rapidly most sea animals, all except the mi 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 numerous 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 (Branehipus, etc.), is to dwarf them. Thus, the little brine-shrimp (q.v.) is apparently a dw a I 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 labora- tory. The lakes near the Sea of Aral are known to vary in decree of saltness at different seasons of the year: the result ot 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 distincl So with (he cockle-shells; the semi-fossil ones on the border of lakes which once formed a part of the Aral Sea vary greatly.
