FIG. 6.--CORAL SNAKE (_Lampropeltis_).] [Illustration: FIG. 279.--GILA MONSTER (_Heloderma suspectum_), of Arizona. If poisonous, it is the only instance among lizards. It is heavy-built, orange and black mottled, and about 16 inches long. Compare it with the green lizard (Fig. 280).] [Illustration: FIG. 280.--CHAMELEON (_Anolis_), or green lizard of southern U.S. Far excels European chameleon (Fig. 281) and all known animals in power of changing color (green, gray, yellow, bronze, and black).] =Survival of the Fittest=.--The two facts of most far-reaching importance in the history of animals and plants are: (1) _Heredity_; animals inherit the characteristics of their parents. (2) _Variation_; animals are not exactly like their parents. The first fact gives stability, the second makes progress or evolution possible. The climate of the world is slowly changing, and animals must change to adapt themselves to it. A more sudden change of environment (surroundings) of animals occurs because of migration or isolation; these in turn are caused by the crowding of other animals or by the formation or disappearance of geographical barriers, such as deserts, water, mountain chains. [Illustration: FIG. 281.--CHAMELEON OF SOUTHERN EUROPE.] The young vary in many ways from their parents. Some have a more protective color or form, sharper claws, swifter movements, etc. The individuals possessing such beneficial variations live longer and leave more offspring, and because of heredity transmit the desirable qualities to some of their young. Variations which are disadvantageous for getting food, defense, etc., cause shorter life and fewer offspring. Thus the _fittest survive_, the unfit perish; an automatic _natural selection_ occurs. Darwin taught that variations are infinitesimal and gradual. Recent experiments and observations seem to show that many variations are by sudden jumps, somewhat resembling so-called “freaks of nature.” As to whether these “sports,” or individuals with new peculiarities, survive, depends upon their fitness for their environment. “Survival of the fittest” results from this natural selection, but the selection occurs between animals of marked, not infinitesimal, differences, as Darwin taught. Darwin’s theory is probably true for species in the usual state of nature; the new theory (of De Vries) is probably true for animals and plants under domestication and during rapid geographical changes. [Illustration: FIG. 282.--EMBRYO OF A TURTLE, showing four gill slits. (Challenger Report.)] =Table for Review= (for notebooks or blackboards).