STERILITY OF HYBRIDS 245 



and that it or they must be supposed to be lost. This conclusion 

 will recall the similar problem raised by the Oenothera mutants 

 (Chap. V) ; and unsatisfactory as it may be to have recourse 

 to such hypotheses we must remember the possibility that as a 

 consequence of hybridisation, subsequent segregation and re- 

 combination of factors, species may have thus actually, as we 

 may say, exploded, and left nothing but a polymorphic group of 

 miscellaneous types to represent them in posterity. If this way 

 of regarding the phenomena be a true one, the sterility now seen 

 when some of the group are re-crossed, becomes analogous to 

 that " reversion or crossing " which we now so well understand 

 to be a consequence of the recombination of characters separated 

 at some previous point in the history of descent. In the partial 

 sterility of the contemporary hybrid we see this character re- 

 appearing, formed now as it was on the occasion of the original 

 cross, by the meeting of complementary factors. 



Another case that may be mentioned in this connection is 

 that of the crosses between various culinary peas (Pisum sativum) 

 and a peculiar form found by Mr. Arthur Sutton growing os- 

 tensibly in a wild state in Palestine. This Palestine Pea is low 

 growing, rarely reaching 18 inches. It is in general appearance 

 like a small and poorly grown field pea. The stems are thin and 

 rather hard. The most obvious differences which distinguish 

 this from other field peas are the marked serration of the stipules, 

 and the development of pith in the pods. Such pith is often 

 present in the pods of peas more or less, but in the Palestines it 

 is so strongly developed as almost to form a lomentum. Curi- 

 ously enough, though the flowers are purple much as those of 

 ordinary field peas, there is no coloured spot in the axils. On 

 the other hand, the stems have coloured stripes running up 

 from the axils. Though this plant differs so little from domes- 

 ticated peas, all crosses with them either failed, or produced 

 hybrids quite or almost quite sterile. This was Mr. Sutton's 

 experience, and on repeating the experiments with material 

 kindly given by him I found the same result. 12 



In a large series of crosses some seeds died or gave rise to 



11 I also had a few Fi seeds given me by Mr. R. H. Lock. 



