Chap. XVIL CONCLUSION. 123 



nieeum (the sterility of which, varied according to the season), 

 and with the Passiflora alata, which recovered its self-fertility 

 when grafted on a different stock. 



It is interesting to observe in the above several cases the 

 graduated series from plants which, when fertilised by their 

 own pollen, yield the full number of seeds, but with the 

 seedlings a little dwarfed in stature — to plants which when 

 self-fertilised yield few seeds — to those which yield none, 

 but have their ovaria somewhat developed — and, lastly, to 

 those in which the plant's own pollen and stigma mutually 

 act on one another like poison. It is also interesting to 

 observe on how slight a difference in the nature of the pollen 

 or of the ovules complete self- sterility or complete self-fertility 

 must depend in some of the above cases. Every individual 

 of the self-sterile species appears to be capable of producing 

 the full complement of seed when fertilised by the pollen of 

 any other individual (though judging from the facts given 

 with respect to Abutilon the nearest kin must be excepted) ; 

 but not one individual can be fertilised by its own pollen. 

 As every organism differs in some slight degree from every 

 other individual of the same species, so no doubt it is with 

 their pollen and ovules ; and in the above cases we must 

 believe that complete self-sterility and complete self-fertility 

 depend on such slight differences in the ovules and pollen, and 

 not their having been differentiated in some special manner 

 in relation to one another ; for it is impossible that the sexual 

 elements of many thousand individuals should have been 

 specialised in relation to every other individual. In some, how- 

 ever, of the above cases, as with certain Passifloras. an amount 

 of differentiation between the pollen and ovules sufficient 

 for fertilisation is gained only by employing pollen from a 

 distinct species ; but this is probably the result of such plants 

 having been rendered somewhat sterile from the unnatural 

 conditions to which they have been exposed. 



Exotic animals confined in menageries are sometimes in 

 nearly the same state as the above-described self-impotent 

 plants ; for, as we shall see in the following chapter, certain 

 monkeys, the larger carnivora, several finches, geese, and 

 pheasants, cross together, quite as freely as, or even. more 



