428 EVOLUTION AND GENETICS 



The Errors of the Past. Darwin's Theory. In our discussion 

 of the existing theories we have noted various advantages and 

 shortcomings which are a guidepost to future hnes of progress. 

 Darwin's theory has been one of the greatest of all time, yet the 

 investigations of modern biology have cast doubt upon its com- 

 plete soundness. A careful estimate of the criticisms to which it 

 has been subjected leaves the feeling that the theory is sound, 

 but by no means a complete explanation of evolutionary processes. 

 The occurrence of natural selection can scarcely be doubted, but 

 before it can operate it must have definitely useful or harmful 

 heritable variations with which to work and the theory does 

 not even attempt to explain the occurrence of such variations. 

 This does not rob it of scientific value, but reduces the range of 

 its usefulness; it becomes, as we have noted, chiefly a theory of 

 adaptation. 



The Mutation Theory. In connection with natural selection 

 the mutation theory supplies an explanation of the occurrence of 

 such permanent changes as the latter must have to work upon. 

 Some examples of mutations of extreme degree are known which 

 suggest the possibility of even varietal or specific differences 

 appearing suddenly ; mutations may therefore be a sufficient cause 

 of the sudden evolution of species in some cases, but it is evident 

 that gradual evolution is more common. Can mutation be a 

 basis for such change? It is obvious in such species as Drosophila 

 melanogaster that this is at least a possibility, for here we see that 

 a multitude of slight mutations are constantly appearing, none 

 of them regarded as specific or even varietal differences, but many 

 potentially of importance to the individual. Natural selection, 

 acting upon these minute characters, may readily be supposed to 

 eliminate some and preserve others, to the ultimate modification 

 of the species. 



Even when this much is granted the mutation theory, however, 

 we still lack an explanation of the source of evolutionary change. 

 Mutations occur, this nobody denies, but what causes them? 

 We can say that they come spontaneously from the heritage of 

 the species, but what does that mean? An explanation to be of 

 any value must make a thing clear in terms of its more familiar 

 causes or components. Hence the admitted occurrence of muta- 

 tions of both great and small degree is merely the recognition of 

 a process of change without explanation, just as the recogni- 



