Page:The New International Encyclopædia 1st ed. v. 07.djvu/373

* EVOLUTION. 325 EVOLUTION. ease of 1 1 ■ « ■ prawn by Herdman, who also kept a number of specimens of different colors under observation in jars, with various colors of sea- weeds and "i background, and in verj differenl degrees of light. The results showed that the adult animal can change its coloring very thor oughly, though not in a very short space of time. The change is due to alterations in the size and arrangement of the pigment-granules of the chromatophores. II will !»' -.ccn from these lads lliat Ha' won derful hues of tropical fish, with their colored bands, the stripes of snakes, or of the tiger, the spots of the leopard, the gay markings of cater pillars and of butterflies, as well as of spiders, are originally due to changes in light ami shade. This will also apply to many of the protective markings of insects, and other animals; the initial cause or factor is the varying action of the sun's light, though natural selection may act as a secondary factor, those forms or varieties most protected by their special style of colora- tion surviving. Effects of Darkness. On the other hand, I he absence of light, or a life in perpetual darkness, has gravely modified the visual organs of cave animals and those living in the abysses of the sea. (See Cave Animals.) In the case of the blind beetles, crawfish, spiders, myriapods, etc., of caverns, we have the most obvious facts show- ing the direct action of the environment. The members of the fauna of our caves have their eyes variously affected: some are blind, others have vestiges of eyes, and others are completely eyeless. The cause is simply the result of disuse, for natural selection does not operate in such cases. The loss of eyesight and the scanty f I renders the body slender, pale, colorless, while in compensation for the loss of sight, the tactile sense is greatly exalted; the antenna, legs, and other appendages are remarkably loug and slen- der compared with those of their out-of-doors relatives. The whole subject of cave life affords a most instructive example of the effects of the absence of light, the disuse of organs and their different degrees of atrophy, and other remark- able modifications of the body, and of use-inheri- tance, all brought about by the action of the primary factors of evolution, without the inter- vention of natural selection. It is proper to say, however, that Weissman and other ardent liar winians account for the facts by natural selec- tion. niher animals live in holes in the sea-bot- tom, as some blind crustaceans ami fishes, whose eyes in the very young are normal, also toward or at maturity become blind, and perhaps eyeless. The blind tishe^ and crustaceans of the deep sea afford similar instances. It is a significant fact that those animals, notably the fishes, are pro- vided with phosphorescent organs. Effects of Chances of Temperature. If a life in total darkness causes great variation and the origin of new forms adapted to strange con- dilions, so also great changes in temperature, as shown in nature, 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 whii most favoi able to its welfare, and tvors nut rii ion, ami hence grovt i ii mi! mull ipli ' i ion ( in the oi her hand, e tt re of cold ( minimu ami of heat (maximum) are unfavorable and tend i" cause death. Cold and it- equivalent, altitude, tends to dwarf plants, shell-, etc. When pond-snails are transported into a cold region, where the temperature is below the optimum, sexual maturity i- reached before I hi amn tl has attained its full growth, ami there i- thus formed a dwarf race bj simple change ol clin Hence this is the rea-oii why Alpine and Arctic species are of verj mall ize compared with those Of low land- in I he temperate ; iriiniti plant-, mollusks, crustaceans, etc., maj 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 temperatun yarii from 25 to 35° C. (76 ' to 94° l-'.f. A gastropod (Melania tuberculata) lives in the hot springs of Algeria in a temperature of 87° F., and a beetle [Hydrobi/us orbicularis) in the hot springs of Hammam-Meskoutine, with a temperature of 55° C. (130° F.); in • ler portions of the heated stream live- a little fish, and the fresh- water crah (Telphusa fkwiatilis). A small mol- lusk supports a heat of 122° F. in Italy, and another {Neritina thermophila) occurs in a hot spring in New Ireland, with a temperature of 122° and 140° F. Finally, the supportable maximum appears to be 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 Rotifera may withstand even 80° ('.. while Protista live in hot springs far above GO C, and green algae 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 158° F. Very striking experiments have recently shown that varieties and species may be artificially pro- duced by variations of temperature, which in sonic cases are like those in nature. This i- as near an actual demonstration of the evolution of specie- as we can expect to reach. Mr. Wallace remarks that we have never -ecu a new spei formed by natural selection: but in these tempera- ture varieties we see how species have arisen by the direct action of a change in the environment. Sudden changes cause death, but if the change is slow and gradual the animal may he- adapted to or acclimatized in a temperature relatively high. By thus raising the tempei ture Dallinger practically pro, lined a new tem- perature race or variety of infusorian (Hetero- mita). 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 year- of experimentation to raise the temperature to 158° F. Beginning with the normal temperature of the water at C0° F., in four months he had raised it to 7(1°. without, however, affecting the monads, which continued to multiply by fission as vigorously as before. When 73° was reached, however, an ad- verse influence seemed to be excited on the organ- isms as regards their vitality and product iveness ; but by keeping the temperature constant for two months the new generations became, so to speak,