NEW COMBINATIONS OF GENES 23 



But for the production of new types that are permanent, 

 that bequeath their peculiarities to their descendants, this 

 method of operation suffers under a very great limitation. 

 This is because the interaction that produces the new charac- 

 teristics occurs between the products of the genes in the 

 cytoplasm, not between the genes themselves. When repro- 

 duction occurs, all these cytoplasmic gene products are lost 

 with the death of the body ; only the unaltered reserves, only 

 the unmixed genes themselves, go into the germ cells that 

 produce the new generation. And further, in reproduction 

 from two parents, the combination of genes that gave origin 

 to the new characteristics is taken apart and the genes are 

 recombined in new ways, in the next generation. Thus the 

 new types, with their perhaps superior characteristics, are 

 destroyed as types, through the same process that produced 

 them. The new or superior types so produced are taken apart 

 at each reproduction; their components are scattered and 

 combined again to form other types. No type produced by 

 recombination of genes is permanent, so long as reproduction 

 from two parents occurs in every generation. 



But in many organisms, including the higher plants and 

 many lower animals, reproduction from two parents is not the 

 invariable method. Reproduction may take place by the 

 division of single individuals. This gives a method of perpet- 

 uating the new characteristics and new combinations that 

 have been produced in the way we have described. In such 

 reproduction from a single parent, the existing combination 

 of genes is not taken apart. Each gene divides, half of it 

 passing into the cells of each of the new individuals. All the 

 individuals produced by this method of uniparental repro- 

 duction have therefore the same combination of genes; a 

 combination identical with that in the original parent. A 

 great number, an entire stock or race, is produced, all having 



