Conclusion 33 



from six different kinds of willows, so should it be pos- 

 sible to combine, by crossing, the hereditary qualities of 

 several individuals into one. 



In the preceding paragraphs we have seen how the 

 single hereditary characters occur as independent units 

 in the experiments of hybridization and crossing, and how 

 they can be attained in almost every degree. In the same 

 way, evidently, must we think of those units as inde- 

 pendent in the ordinary process of fertilization as well. 



7. Conclusion 



Seemingly elementary, the specific character is ac- 

 tually an exceedingly complex whole. It is built of nu- 

 merous individual factors, the hereditary characters. The 

 more highly differentiated the species, the higher is the 

 number of the component units. By far the most of these 

 units recur in numerous, many of them in numberless or- 

 ganisms, and in allied species the common part of the 

 character is built up of the same units. 



On trying to analyze species into these individual 

 factors, we are confused by their number, which, in the 

 higher plants and animals reaches probably into the 

 thousands. If, however, we regard the entire world of 

 organisms as the subject of our analysis, then the total ^ 

 number of hereditary characters which is needed for the 

 building up of all living beings, is indeed large in itself, 

 but, in relation to the number of species it is small. In 

 that limited sphere our method of investigation leads ap- 

 parently only to complications, but, on the whole, it evi- 

 dently leads the way towards a very considerable simpli- 

 fication of the problems of heredity. 



The hereditary factors, of which the hereditary charac- 

 ters are the visible signs, are independent units which may 



