EFFECTS OF CHANGES OF TEMPERATURE. If a life in total darkness causes great variation and the origin of new forms adapted to strange eon -ditions, so also great ehanges in temperature, as shown in and by laboratory experiments, afford the strongest circumstantial evidence of the origin of new species by changes in the con dition of life. It should be observed that varia tions due to changes of temperature are not fortuitous, but in direct relation to such changes of environment.
There is for each individual, and hence for each species, an optimum temperature which is most favorable to its welfare, and most favors nutrition, and hence growth and multiplication. (In the other hand, extremes of cold linininulnO and of heat (maximum) are unfavorable and tend to cause death. Cold and its equivalent, altitude, tends to dwarf plants, shell,. etc. When pond-snails are transported into a cold region, where the temperature is below the optitnunt, sexual maturity is reached before the animal has attained its full growth, and there is thus formed a dwarf race by simple change of climate. Ilenee this is the reason why Alpine and Arctic species are of very small size compared with those of lowlands in the temperate zone.
Certain plant,, mollusks, crust aeeans, ete., may become adapted to hot springs. constituting a thermal-spring fauna. Several kinds of mollusks live and prosper in the thermal waters of the Pyrenees, and of Dax, whose temperature varies from 25° to 35° C. (76° to 14° F.). A gastropod (.11clania tubereula(a) lives in the hot springs of Algeria in a temperature of S7° F. and a beetle (Ilydrobius orbicula•is) in the hot springs of with a temperature of 55° C. (130° F.) : in cooler portions of the heated stream lives a little fish, and the fresh water crab (D.114111:01 !US). A small mol lusk supports a heat of 122° F. in Italy, and another thennophilu) °•mi•s in a hot spring in New treland, with a temperature of 122° and 140° F.
Finally, the supportable maximum appears to he confined between 105° and 113° F. It is known that at 122° F. protoplasm, at least in ver
tebrate animals, partially coagulates, and this causes death as by sunstroke, though notifera nay withstand even S0° C., while Protista live in hot springs far above (i0" C., and green alga can survive 70° C. Yet, as we shall see further on. monads can be so modified by a gradual eleva tion of the temperature as to withstand the ex treme of 15S° F.
Very striking experiments have recently shown that varieties and species may he artificially pro duced by variations of temperature, which in some cases are like those in nature. This is as near an actual demonstration of the evolution of species as we can expect to reach. Sir. Wallace remarks that we have never seen a new species formed by natural selection ; hut in these tempera ture varieties we see how species have arisen by the direct action of a change in the environment.
changes cause death, but if the change is slow- and gradual the animal may heroine adapted to or acclimatized in a temperature relatively high. By thus raising the tempera ture Dallinger practically produced a new tem perature-race or variety of infusorian (Iretero mita 1. For a period of over ten years he made observations on this infusorian. Observing that a new generation comes into existence every four minutes or so. it took years of experimentation to raise the temperature to 15S° F. Beginning with the normal temperature of the water at GO° F., in four months he had raised it. to 70°, without, however, affecting the monads. which continued to multiply by fission as vigorously as before. 73' was reaehed, however, all ad verse influence seemed to be excited on the organ isms as regards their vitality and produetiveness: but by keeping the temperature constant for two months the new generations became, so to speak, acclimatized, and in five months more the tem perature was gradually raised to 78°. These experiments were continued until the tempera ture of 15S° F. was reached, when an accident put an end to the experiments, and the new race thus adapted became extinct.