550 



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 with- 

 out 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 observa- 

 tion and may be supplemental to 

 any of the others. 



A second method of evolution 

 is mutation. Mutation may be 

 defined as a sudden hereditary 

 change in the appearance of an 

 animal type ; the term was originally 

 applied especially to those changes 

 which were striking, but any minute 

 hereditary change is a mutation. 

 Slight mutations are not clearly 

 distinguishable from the continuous 

 variations which are assumed in the 

 theory of natural selection. Sup- 

 port 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 characteristics. DeVries, who was the 

 author of the theory that evolution was due to mutation, based it upon 

 experimental work with evening primroses. 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 (Fig. 379). 



Another method of evolution was suggested by Lotsy. He believed 

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



Fig. 379. — An albino crow, Corvus b. 

 hrachyrhynchos Brehm, an example of a muta- 

 tion which arose spontaneously and in which 

 all pigment is absent. 



