In a large series of crosses some seeds died or gave rise to feeble plants. Of the plants which lived, few gave any seed. The seed, however, that was obtained from F_{ 1} 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 experiment 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.
1. 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 experimental 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.