326 EVOLUTION, GENETICS, AND EUGENICS 



the mutation theory is that the one supposes these varieties to arise 

 through selection of individual variations, the other supposes that 

 they have arisen spontaneously and at once from the original form. 

 The development of these varieties into new species is again sup- 

 posed, on the Darwinian theory, to be the result of further selection, 

 on the mutation theory, the result of the appearance of new muta- 

 tions. 



In consequence of this difference in the two theories, it will not 

 be difficult to show that the mutation theory escapes some of the 

 gravest difficulties that the Darwinian theory has encountered. 

 Some of the advantages of the mutation theory may be briefly 

 mentioned here. 



i. Since the mutations appear fully formed from the beginning, 

 there is no difficulty in accounting for the incipient stages in the 

 development of an organ, and since the organ may persist, even when 

 it has no value to the race, it may become further developed by later 

 mutations and may come to have finally an important relation to the 

 life of the individual. 



2. The new mutations may appear in large numbers, and of the 

 different kinds those will persist that can get a foothold. On account 

 of the large number of times that the same mutations appear, the 

 danger of becoming swamped through crossing with the original form 

 will be lessened in proportion to the number of new individuals that 

 arise. 



3. If the time of reaching maturity in the new form is different 

 from that in the parent forms, then the new species will be kept from 

 crossing with the parent form, and since this new character will be 

 present from the beginning, the new form will have much better 

 chances of surviving than if a difference in time of reaching maturity 

 had to be gradually acquired. 



4. The new species that appear may be in some cases already 

 adapted to live in a different environment from that occupied by the 

 parent form; and if so, it will be isolated from the beginning, which 

 will be an advantage in avoiding the bad effects of intercrossing. 



5. It is well known that the differences between related species 

 consist largely in differences of unimportant organs, and this is in 

 harmony with the mutation theory, but one of the real difficulties of 

 the selection theory. 



6. Useless or even slightly injurious characters may appear as 

 mutations, and if they do not seriously affect the perpetuation of the 

 race, they may persist. 



