556 THE CAUSES OF EVOLUTION 



the fittest. As it now stands, selection is the most satisfactory- 

 explanation that can be given of the adaptations that are present 

 in organisms (cf. p. 24). The alternative Lamarckian Theory 

 has scant foundation. In this connection it is also said that selec- 

 tion can act only upon differences that are of marked importance, 

 and most characteristics of animals are not on this level. How 

 such non-selective characters may become established in a race is 

 shown in the sections upon Mutation and Isolation. 



Again, it is said that no one has seen selection at work in nature, 

 and that we merely argue, because of the facts, that it must work 

 {cf. Figs. 305 and 306). This only means that nature is too com- 

 plex to show all that happens. We see dimly and incompletely, but 

 none the less certainly, the existence of a selective process which is 

 always in the background even though it may be temporarily in 

 abeyance. This is all that one can expect to see in so complex a 

 situation, although special cases of selection may be demonstrated 

 experimentally, as when chicks of different colors are let run in a 

 large enclosure and it is found at the end of the season that the 

 white ones have been picked off by hawks. 



It is true that many examples of variation that Darwin cited in 

 his argument for Natural Selection are now believed to be fluctua- 

 tions rather than mutations and therefore not heritable. In Dar- 

 win's day, extreme examples of mutation, or " sports " as they 

 were called by breeders, had been recognized; but Darwin 

 regarded such variations as too infrequent to be important. At 

 the present time we know mutations to be of frequent occurrence 

 and to include all sorts of differences. The argument for selection 

 is sound so long as there are such heritable variations, that is, 

 mutations, even though Darwin had in mind principally variations 

 that are now known to be fluctuations and therefore not 



inherited. 



Perhaps the strongest argument for Natural Selection is based 

 upon the results obtained by Artificial Selection. Varieties of 

 domesticated animals and plants have been developed by the arti- 

 ficial selection of individuals having variations that pleased the 

 fancies or the necessities of breeders. Insofar as these variations 

 are inherited, they may thus be perpetuated and the breed modified. 

 The breeder does not even need to know the special laws of heredity 

 and variation in order to accomplish notable results, although such 

 knowledge enables him to select much more effectively. Arti- 



