Intergrades of Independent Derivation 229 



Excellent illustrations are furnished by Professor Thomas H. 

 Morgan from his work with the fruit flies. Among the 

 hundreds of mutations that have arisen in the cultures are 

 individuals with various modifications of the wings, of the 

 eyes, of the legs, and of the abdomen. A series of flies can 

 be arranged whose eyes vary by slight gradations from white, 

 through pink, to brown. Such a graded series suggests the 

 ** evolution " from one extreme to the other. As a matter 

 of fact, however, these eye types appeared at various times 

 without any connection between the position in the series 

 and the position in the time sequence. Moreover, these vari- 

 ous types did not arise from one another in any definite or- 

 der but all came from the parental or wild type, mutations 

 having thus appeared independently both in the direction 

 of less pigment and in the direction of more pigment. 



Intergrades of Independent 'Derivation 



A series of flies may be arranged in the order of wing 

 size, from very long at one extreme to the merest stumps of 

 wings at the other. Such a series would again suggest the 

 gradual evolution from one extreme to the other. Here too 

 the distinct types appeared at various times without any re- 

 lation between the position of a type in the size series and 

 its position in the time sequence. The wild type of fruit 

 fly was in every case the parental form from which each of 

 these modifications of the wings arose. There are other de- 

 partures from the orthodox wing type, such as different 

 degrees of bending or curling. In this series also each of 

 the distinct types has arisen independently without definite 

 time relationship to the origin of the others. (See also 



Fig- 99') 



The continuity of a series is thus not an indication of the 



order in which the various characters appeared. There is 

 nevertheless in these experimental studies, and in those of 

 other workers, clear evidence as to the plasticity of the liv- 

 ing stock, its capacity to produce new variants in many 



