STERILITY OF HYBRIDS 247 



2. We may follow the conventions of systematists and dis- 

 tinguish the outstanding or conspicuous forms such as arvense, 

 quadratum, sativum and perhaps a few more as species, and leave 

 the rest unheeded. If this course is followed it must be clearly 

 understood and permitted as a piece of pure pragmatism, deliber- 

 ately adopted for the convenience of cataloguers and collectors, 

 without regard to any natural fact or system whatsoever. 



But while following either the one plan or the other we shall 

 be still awaiting the answer, which only genetic experiment 

 can provide, to the question whether among the various types 

 there are some which differ from the rest in a peculiar way: 

 whether by having groups of characters linked together in 

 especially durable combinations, or by possessing ingredients 

 which cause greater or less disturbance in the processes of cell- 

 division, and especially in the processes of gametic maturation, 

 when they are united by fertilisation with complementary 

 ingredients. 



Before any but the vaguest ideas regarding the nature and 

 significance of inter-specific sterility can be formed, a vast 

 amount of detailed work must be done. Sterility as a result of 

 crossing, as well as that which is alleged sometimes to arise in 

 consequence of changed conditions, is at best a negative charac- 

 teristic, and there are endless opportunities for mistake and mis- 

 interpretation in studying features of this kind. No one, I 

 suppose, would now feel any great confidence in most of the data 

 which from time to time are resuscitated for the purpose of such 

 discussions. Even the best collections of evidence, such as those 

 given by Darwin in Forms of Flowers, cannot be regarded as 

 critical when judged by present-day standards. Nothing short 

 of the most familiar acquaintance with the habitual behaviour 

 of individuals, and of strains kept under constant scrutiny for 

 several years would enable the experimenter to form reliable 

 judgments as to the value to be attached to observations of 

 this class. 



The admission must, however, be faced that nothing in recent 

 work materially tends to diminish the surprise which has always 

 been felt at the absence of sterility in the crosses between co- 



