326 LUTHER BURBANK 



tions earlier. But the influence of the celebrated 

 experimenter, Carl Friedrich von Gaertner, 

 had served to give credence to the opposite 

 opinion. 



Darwin had argued for the fertility of some 

 natural hybrids, but he had not been able to 

 make out a case that by any means carried con- 

 viction to the generality of biologists and bota- 

 nists; and the current opinion was that the com- 

 paratively few cases of the fertility of seeming 

 hybrids might best be explained either on the 

 supposition that the observed forms were not 

 really of the parentage ascribed to them; or else 

 that the parent forms, even though classified as 

 different, were not really entitled to rank as 

 independent species. 



In a word, the doctrine of Kolreuter and his 

 followers, which would make the sterility of the 

 hybrid offspring a test of the specific diversity of 

 the parent forms, was perhaps the stock doctrine 

 of the biological world. 



The implications of such an argument are 

 obvious. If we are to answer the question, 

 "What is the test as to whether two forms are 

 entitled to recognition as different species?" by 

 saying, "They are different if their hybrid off- 

 spring are sterile, and they are only varieties if 

 their offspring are fertile" we should obviously 



