. Animal biology. Zoology; Biology. EVOLUTION OF ANIMALS 551 may be developed constantly as a result of hybridization, not in the sense of crossing two species but in the sense of crossing genetic characters. Still another method is that known as orthogenesis. Paleontologists have been very active in urging the importance of this as a method and the evidence to support it comes largely from fossil types. According to the theory of natural selection, w^hen a characteristic becomes harmful to an animal it should disappear, but there is paleontological evidence to the effect that in the past many
. Animal biology. Zoology; Biology. EVOLUTION OF ANIMALS 551 may be developed constantly as a result of hybridization, not in the sense of crossing two species but in the sense of crossing genetic characters. Still another method is that known as orthogenesis. Paleontologists have been very active in urging the importance of this as a method and the evidence to support it comes largely from fossil types. According to the theory of natural selection, w^hen a characteristic becomes harmful to an animal it should disappear, but there is paleontological evidence to the effect that in the past many types that have speciaHzed in certain directions have gone on developing in that direction, even when over- specialization has resulted in harm and has ultimately led to the extinc- tion of the animal. An example of such a type is the saber-toothed tiger (Fig. 380); its upper canines developed until they became exceedingly effective both in the securing of prey and in defense, but they seem to. Fig. 380.—Saber-toothed tiger; restoration. {Redrawn from Scott, "History of Land Mammals in the Western Hem,isphere," by the courtesy of The Macmillan Company.) have gone on developing until they became a handicap and perhaps were ultimately a factor in its extinction. Another animal often given as an example of overspecialization is the Irish elk the antlers of which were greatly developed; at first this was to the advantage of the animal but later they reached such a size as to impede its progress in the forests and place it at a disadvantage in escaping from enemies. To explain such cases as this it has been suggested that the development of a character is due to a hereditary tendency accompanying a progressive change in the genes, w^hich causes the animal to develop constantly in a certain direc- tion. If the result is to make the animal more effective, natural selection tends to perpetuate the type, but when overdevelopment and disadvan- tage follow, the hereditary ten
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