306 FOUNDATIONS OF BIOLOGY 



separating one line from another, as is attested by practical 

 breeders everywhere, and in taking advantage of mutations 

 when they occur. Most of the 'new creations' in horticulture 

 and animal breeding are the result of hybridization and the 

 rigid selection of individuals exhibiting desirable new com- 

 binations and sometimes mutations which hybridizing seems 

 to induce. Natural selection, in a quite similar manner, may 

 act as a 'sieve' and sort out new combinations and mutations 

 presented leave the fit and eliminate the unfit and so 

 afford a natural explanation of the adaptation of organisms 

 to their environing conditions. (Fig. 194.) 



SUMMARY 



Before leaving the subject a brief summary of the most 

 important general principles which the study of genetics has 

 thus far afforded may be helpful. In the first place, it appears 

 clear that the basis of inheritance is in the germinal rather 

 than in the somatic constitution of the individual. A charac- 

 ter to be inherited must be innate in the germ cells, and there 

 is no satisfactory evidence that modifications of the body, 

 'acquired characters,' can be transferred to the germ and so 

 inherited. Secondly, characters or groups of characters are 

 usually, if not universally, inherited as definite units. These 

 follow Mendelian principles of segregation and recombination 

 in the formation of the germ cells of an individual, so that 

 paternal and maternal contributions are readjusted in all the 

 combinations which are mathematically possible. And 

 finally, the germinal factor basis (genes) of unit characters is 

 remarkably constant. Selection is apparently powerless to 

 alter it, but merely sorts out what is already there, or, taking 

 advantage of such changes (mutations) as do occur, deter- 

 mines their survival value for their possessor in the struggle 

 for existence. 



