52 EVOLUTION OF LIVING ORGANISMS. 



in sets by individual chromosomes in the gametes, and 

 further it is claimed that the actual position of the 

 factors in the chromosomes can be determined. 



The conclusion is that the total inheritance transmitted 

 in a gamete may be interpreted as made up of definite 

 independent unit factors contributing to the develop- 

 ment of corresponding characters. While the factors 

 are either entirely absent or present, the characters due 

 to them may be more or less developed according to inter- 

 action with other factors of inheritance and with factors 

 of the environment. The origin of at all events the 

 majority of domestic races of animals and plants is ac- 

 counted for on the theory that the complex of factors of 

 inheritance possessed by the primitive wild form has been 

 split up among a number of races, each distinguished by 

 the loss of some factor or set of factors belonging to the 

 original stock. The various domestic races of pigeons, 

 fowls, sweet-peas, and so forth, would differ from each 

 other in holding a different selection from out of the 

 whole number of available original factors. If this were 

 true we might expect to be able to bring together again 

 the separated fragments of inheritance and reconstruct 

 the complete set by crossing various races with the neces- 

 sary elements. And this can indeed be done, giving rise 

 to "reversion," the reappearance of an ancestral com- 

 bination of characters. Thus the original wild form of 

 sweet-pea found in Sicily can be reconstructed by cross- 

 ing the " Bush " and the " Cupid " domestic varieties. 

 The "Agouti" colour of the wild rabbit reappears on 

 crossing a yellow with a Himalayan variety ; and the 

 plumage of the wild " blue rock pigeon " (Columba livia) 

 can be reproduced on crossing certain very unlike domestic 

 races. 



But do all factors of inheritance "mendelise"? In 

 answer to this question it may be said that the law of 

 segregation, the law of Mendel, appears to hold good with 

 all sorts of characters and in all sorts of plants and 



