No. 596] SHORTER ARTICLES AND DISCUSSION 495 



made by East,'' and (Enothera studied by Geerts. 1 " But in these 

 cases we are dealing with species hybrids, with plants in which 



cause. In the case of Xicotiuita the parent plants wnv both 

 (one certainly, the other in all probability) self-sterile and thus 

 the cause of self -sterility in the cross-bred offspring is explained. 

 In Stizolobium Belling found, as Geerts had previously noticed in 

 (Enothera, that in the hybrids a certain proportion of both ovules 

 and pollen grains were shriveled and malformed. Here again 

 the cause of the sterility is plain. If, as Frost suggests, the ever- 

 sporting single stock should be regarded as a hybrid, it differs 

 completely from the cases referred to above, for in this case there 

 is no obvious sign or cause of sterility either in the plant itself or 

 in the pure-breeding single from which we suppose it to have 

 arisen. It becomes then a question whether to attach weight to 

 the argument from analogy in a case where it goes against all the 

 evidence available. 



From the considerations here reviewed I am led to sum up the 

 position as follows : 



Evidence is wholly lacking in the stock itself in support of 

 Frost's hypothesis that the behavior of the ever-sporting 

 (double-throwing) single is to be accounted for as the result 

 of the selective sterility of ovules and pollen. Not only so, but 

 it may be claimed that the facts on which he relies to support 

 his argument can equally well be adduced in favor of the opposite 

 point of view. The selective elimination of embryos or the more 

 frequent fertilization of egg cells of s as compared with those of S 

 (or SJ constitution seem both untenable in view of the fact that 

 the usual excess of doubles is obtained in eases where every ovule 

 is fertilized and every resulting seed germinates. In the formal 

 scheme which I have "put forward we have a working hypothesis 

 which enables us to correlate the present known facts. Does 

 Frost's hypothesis give us more than this? Are we not, in the 

 end, still left debating whether his various speculations unsup- 

 ported by facts really carry us further, and whether they can 

 justly lay claim to the merit of greater simplicity? 



