344 PRINCIPLES OF ANIMAL BIOLOGY 



numerous differences among organisms belonging to the same line of 

 descent; but in large part these diff(;rcnces are due to the environment 

 and in the main are probably not inherited. Such non-heritable dif- 

 ferences are known as fluctuating variations, a,nd have no influence upon 

 evolution. Occasionally, however, new characters appear which are 

 inherited. These are called mutations, early knowledge of which was 

 largely due to Hugo de Vries, and every mutation is a step in evolution. 



Witnessing evolution thus consists in discovering mutations as they 

 occur. This has been done in a considerable number of instances. One of 

 the most fruitful sources of mutations has been the small fruitfly Drosoph- 

 ila extensively studied by T. H. Morgan and others. Owing to the 

 ease with which this fly is reared in confinement, upon fermented bananas, 

 it has been used much for experiments. At first its value as a means of 

 studying evolution was not fully appreciated. It was bred in the labora- 

 tories of several universities for the purpose of testing the effectiveness 

 of selection and the influence of the environment, and for experiments 

 upon fertility, length of life, and the like. Then mutations began to 

 appear. It is possible that mutations had been appearing at intervals 

 previously, but if so they did not attract attention. At any rate, mu- 

 tations so striking that they could not be overlooked occurred. One 

 was a change from the normal red eye of the wild fly to a white eye. 

 Since the flies were examined with a microscope, it is not Hkely that any 

 such striking alterations escaped observation. The one fly with white 

 eyes was probably the first that had been produced since the stock of 

 flies in which it appeared had been taken into the laboratory. Moreover, 

 when this white-eyed fly was bred with others, the absence of red pig- 

 ment in the eye was definitely inherited as a recessive trait. Mendelian 

 rules were clearly followed, and the case of white eye in flies might have 

 been used as a simple example of heredity in the chapter on Genetics, 

 instead of albinism in guinea-pigs, were it not for the fact that the posses- 

 sion of white eyes is sex-linked. White eye in Drosophila, then, is 

 a new characteristic which is inherited. It is therefore a mutation and 

 a step in evolution. There are now many white-eyed fruit flies — in 

 captivity. Perhaps, had the first white-eyed fly occurred in nature, it 

 would have perished. Nevertheless, its occurrence would have been a 

 case of evolution. The effect of the death of individuals in which muta- 

 tions have occurred is discussed later. 



Other changes have occurred in the same species of Drosophila (see 

 Fig. 236), Eye colors other than white, to a few of which the names eosin, 

 vermilion, cherry, buff, and tinged have been given, have appeared. New 

 shapes and sizes of wings have occurred. In one the wings were consid- 

 erably shortened, though they were still functional; in another, the wings 

 were greatly reduced and of abnormal shape, so as to be useless for flight; 

 instill another the wings were inflated hke a balloon; in a fourth, wings 



