3IO O. F. COOK 



WHY MUTATIONS ARE RECESSIVE. 



That inbreeding induces many of the evolutionary aberra- 

 tions of domestic plants and animals is shown by the fact that 

 such characters commonly disappear in crosses with the sym- 

 basic, or freely-interbred, wild type. Darwin's classical experi- 

 ments with pigeons have been repeated and supplemented by 

 many observers in Europe and America, and additional testi- 

 mony of the same kind has been published recently by Professor 

 Castle.' The "Angora coat" and other similar abnormalities 

 of inbred animals are found to be recessive, in the Mendelian 

 sense ; that is, the long hair disappears when crossed with the 

 short. 



Whether such characters are " recessive " or " dominant," or 

 whether they appear at all, may depend on the relative degrees 

 of inbreeding, rather than upon any special strength or weak- 

 ness of characters as such. Like normal genetic variations, 

 mutations are prepotent with their own equally inbred relatives, 

 but abnormalities induced by inbreeding can be corrected when 

 more remote lines of descent are brought together. Professor 

 Castle maintains that to preserve such mutations as the long- 

 haired guinea-pigs and horses they must be bred with others of 

 like kinds, but in accordance with the present interpretation it 

 will be found more effective to continue inbreeding with their 

 own immediate, unmutated relatives. The fact that these long- 

 haired mutations arise in the first place from short-haired parents, 

 should not be overlooked. 



Plant mutations which can be propagated asexually or by 

 self-fertilization are often remarkably constant. With animals 

 the experiment is more difficult because some crossing, at least 

 of individuals, is necessary to reproduction. The remote clrance 

 that mutations sometimes initiate new^ species would be still 

 further attenuated if it were necessary that two of the same kind 

 arise at the same time and place in order to make possible the 

 preservation of the new type. 



Under the kinetic theory- no fundamental importance is 



'The Ilcrcditj of "Angora" Coat in Mammals, Science, N. S., iS: 760, 

 1903. 



^ A Kinetic Theory of Evolution, Science, N. S., 13 : 969, June 21, 1901. 



