Chapter XI 



THE ORIGIN OF SPECIES BY 

 NATURAL SELECTION 



The question still remains whether under natural 

 selection the mutational changes that appear spo- 

 radically will suffice to sujjply the materials for new 

 species. Genetics has shown that in all probability 

 only one gene of a pair mutates at a time. If the 

 mutation occurs late in the history of the germ-cells, 

 the mutated gene might be retained in an egg or in 

 two sperm-cells. If the mutation occurred earlier in 

 the germ track the mutated gene might, if in a fe- 

 male, remain in several eggs after the polar bodies 

 are formed, or if in a male, remain in many sperm- 

 cells. If a germ-cell carrying the new gene happens 

 to combine with a germ-cell of another normal indi- 

 vidual, the cells of the embr^^o so produced will con- 

 tain the gene in only one of its chromosomes, and 

 such an individual will not show the character if the 

 gene is recessive. In this individual, half of the 

 mature germ-cells will now contain the gene and 

 half its normal partner. If such an individual mates 

 with a normal individual, half of its offspring will 

 carry the gene in only one chromosome of each cell. 

 Here, for the first time, the new gene is present in 

 many individuals — in half as many as are produced 

 by the mating. This process may hy chance be re- 



