522 GENERAL CONSIDERATIONS 



preserved and harmful ones eliminated. (5) Such changes are passed 

 on from generation to generation and result in a gradual change in the 

 character of the species. (6) If upon being dispersed into other regions 

 individuals can escape the effect of the struggle for existence or exist 

 under altered conditions, they may develop characteristics different 

 from those they have previously possessed and this may result in the 

 production of a new species. Such species usually show definitely their 

 relationship to the parent species. Natural selection seems to be without 

 doubt one means by which changes may be passed on. Though it is 

 not the only method it is the one which is supported by the most extensive 

 evidence from observation and may be supplemental to any of the others. 



A second method of evolution is mutation. Mutation may be defined 

 as a sudden change in the appearance of an animal type; the term was 

 originally applied especially to those changes which were striking, but 

 any evident change is a mutation. Slight mutations are not clearly 

 distinguishable from the continuous variations which are assumed in the 

 theory of natural selection. Support for the theory of mutation as a 

 method of evolution is seen in the appearance in nature of so-called sports 

 — new types which have suddenly appeared and which have transmitted 

 their characters to succeeding generations. Many cases of mutation 

 in the fruit fly are known, which involve eye color, the shape and size of 

 wings, body color, additional bristles, and other less obvious character- 

 istics. DeVries, who was the author of the theory that evolution was 

 due to mutation, based it upon experimental work with evening prim- 

 roses. The suggestion has been made that he was dealing with hybrids 

 and not with pure forms and that thus the types which he produced did 

 not, at least in part, represent true mutations. Nevertheless this does 

 not explain all of his results, and numerous examples of mutation are now 

 known in both plants and animals. 



Another method of evolution was suggested by Lotsy. He believed 

 that since all animals are from the genetic standpoint impure, new types 

 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, when 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 specialized 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. 319); its upper canines developed until they became exceedingly 

 effective both in the securing of prey and in defense, but they seem to 



