246 PROBLEMS OF GENETICS 



feeble plants. Of the plants which lived, few gave any seed. 

 The seed, however, that was obtained from FI plants grew well 

 enough, and the F 2 plants proved, as often in such cases, fertile. 

 In these, indeed, no sign of sterility was noticeable. The experi- 

 ment is being repeated in various ways, for, as the genetic 

 behaviour of peas is comparatively well known, the subject is an 

 exceptionally favourable one for these investigations. 



Such an example shows the confusion produced the moment 

 we attempt to harmonize conceptions of specific difference with 

 results attained by experimental methods. It has been usual 

 to regard the field pea (P. arvense) as a species distinct from the 

 edible pea (P. sativum). De Candolle and others regard the 

 field pea as derived from a form wild in Italy, but the origin of 

 the edible pea is considered to be unknown. From breeding 

 experiments we find no sterility whatever in the crosses between 

 the various arvense and sativum types, nor in the crosses made 

 between them and several other peculiar types from various 

 countries; whereas this Palestine Pea, which only differs from a 

 small arvense in what might have been thought trivial characters, 13 

 either fails to cross altogether or gives a sterile product, whatever 

 type be chosen as the other parent. 



Examples of this kind have at least the merit that they lead 

 to more precise delimitations of the problem. We are confronted 

 with two distinct alternatives. 



i. We may apply the term Species promiscuously to all 

 distinct forms. If we do so it must be clearly understood that 

 we cannot even rule out the several combinations of " presences 

 and absences " represented by the various types whether wild or 

 domesticated. For we may feel perfectly assured that at least 

 all the arvense and all the sativum types yet subjected to experi- 

 mental tests are on precisely the same level in this respect. There 

 is no distinction, logical or physiological, to be drawn between 

 them. Some contain more factors, and others contain fewer. 

 In some the re-combinations have been brought about by natural 

 variation or crossing, while the same consequences in the others 

 have resulted from man's interference. 



u In a paper about to appear in Jour Linn. Soc. Mr. A. W. Button identifies 

 this Palestine pea as Pisum humile of Boissier and Noe. 



