274 



EVOLUTION AND GENETICS 



in the minuteness of the matter from whicli the new individual 

 develops. This consists of a single cell, sometimes formed by 

 the union of two independent germ cells derived from opposite 

 sexes. 



The Hereditary Bridge. Whatever the source of the single 

 cell which gives rise to a new individual, it constitutes a link 



between generations which is of 

 the greatest importance to the 

 geneticist. All hereditary parts of 

 an organism must necessarily come 

 from its parents. In many cases 

 the production of the original germ 

 cells is the sole connection of the 

 parents with the new generation. 

 Obviously then everything which 



Fig. l^S.-Hydra reproducing appears in the developing individ- 

 asexually by dividing length- ual must be based upon something 

 wise. (From Woodruff, after present in the cell from which it 



arises. It is the most compact ex- 

 pression of the complete organism and well deserves Walter's 

 appellation, the hereditary bridge. 



The value of studies of the behaviour of germ cells was recognized 

 long ago and cytological investigations correlated with observa- 

 tions of the behaviour of characters in heredity are now a funda- 

 mental part of genetics. 



Hybridization. Unscientific breeders have not been alone in 

 utilizing this process. Crossbreeding of different strains within 

 a species is now the most important source of material for the 

 study of inheritance, for in the sharp contrast of characters thus 

 obtained the behaviour of any one is usually conspicuous. 



Hybridization demands first of all, known strains or varieties, 

 which are best secured by careful breeding under observation to 

 determine the normal course of their heredity. Once isolated, 

 strains with contrasting characters can be crossed at will, and in 

 the phenomena of reproduction through successive hybrid genera- 

 tions is found the source of important laws of heredity. 



Selection. In the establishment of pure strains for hybridiza- 

 tion selection is a necessary part of the methods of genetics. It 

 has been important in other ways, but in this science it is chiefly 

 an accessory of hybridization. 



