1915^ Outline ok the Theory of Descei^t 19 



hybrids, and curiously enough this fact was invoked as long 

 ago as 1837 in support of the view that the sperm was really 

 essential to the act of fertilization. In that year R, Wagner 

 showed that sperms were invariably absent in non-fertile 

 hybrids and as invariably present in all fertile males. Jeffrey 

 found that without exception a high degree of sterility was 

 characteristic of every species examined. Furthermore all 

 known hybrid plants available were examined with essen- 

 tially the same result. On the other hand an extensive in- 

 vestigation, ranging from the highest to the lowest plants and 

 including a large number of species as to whose genetic 

 purity no suspicions are entertained, revealed absolutely no 

 sterile pollen whatever. Since in this respect the evening 

 primroses ally themselves with undoubted or suspected hy- 

 brids Jeffrey maintains that the group as a whole is much 

 contaminated by hybridization and that no such important 

 theory as mutation should rest upon so dubious a foundation. 

 De Vries has recently replied at considerable length to Jeff- 

 rey's attack but without, in my judgment, seriously damag- 

 ing the criticism. 



Among other things, he cites cases of mutation in the com- 

 mon shepherd's purse and asserts that its pollen is all perfect. 

 This, however, is manifestly an error. I have just begun an 

 examination of this plant myself and there is abortive pollen 

 here, just as Jeffrey found in all his hybrids. 



Since the connection between sterility and hybridization 

 has been so long and so well known, it would seem strange 

 that its significance should have escaped De Vries, who was 

 aware of the fact that sterility was more or less abundant in 

 his primroses. In the controversy which is still going on, it 

 appears that De Vries is inclined to admit the contention of 

 Davis, Tower, Jeffrey and others that mutants may arise as 

 a result of hybridization, but still clings, with other muta- 

 tionists, to the view that mutation is a distinct phenomenon 

 in no way dependent on hybridization. 



To be sure there are numbers of cases where mutants or 

 sports have appeared in which it is very difficult to impugn 



