MUTAtlONS AND VARIETIES. 7 1 



16. Mutations and Varieties. 



In a recent volume by Prof. T. H. Morgan (Evolution and 

 Adaptation, 1903) we find an interesting exposition of the mutation 

 theory, from which we quote a few sentences : 



Amongst the mammalia and birds of North America there are many cases of 

 local forms or races, some of which at least are probably mutations. This can 

 only be proven, however, by actually transferring the forms to new localities in 

 order to find out if they retain their original characters or become changed into 

 another form [p. 292]. As De Vries has pointed out, each mutation may be differ- 

 ent from the parent form in only a slight degree for each point, although all the 

 points may be different. The most unique feature of these mutations is the con- 

 stancy with which the new form is inherited. * * * There is another point 

 of great interest in this connection. Many of the groups that Darwin recognized 

 as varieties correspond to the elementary species of De Vries. These varieties, 

 Darwin thought, are the first stages in the formation of species, and, in fact, can 

 not be separated from species in most cases. The main difference between the 

 selection theory and 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 supposed, on the Darwinian theory, to 

 be the result of further selection ; on the mutation theory, the result of the appear- 

 ance of new mutations. * * * gome of the advantages of the mutation 

 theory may be briefly mentioned here. ., 



1. 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 the dan- 

 ger 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 form, 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 ma- 

 turity had to be gradually acquired. 



4. The new species may be in some cases already adapted to live in a different 

 environment. * * * and so will be isolated from the beginning. * * * 



5. It is well known that the difference between related species consists 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 (pp. 

 297-299). 



