( cxv ) 



these injurious results have been not the cause but the con- 

 sequence of specialisation for cross-fertilisation. In such 

 plants fertilisation is mainly brought about along the line 

 for which special adaptation is made : self-fertilisation is 

 relatively infrequent, often very rare, sometimes perhaps 

 absent altogether. May not the less successful i-esults have 

 followed from a condition in which self-fertilisation is but 

 little tried by the fires of selection?* It would be of much 

 interest to compare a long series of experiments on the cross- 

 fertilisation of plants which are habitually self-fertilised, and 

 on the self-fertilisation of plants in which the adaptations 

 for cross-fertilisation are made use of in widely different 

 degrees. 



This criticism, should it be sustained, would of course throw 

 much light upon the case of the Bee Orchis and the nvunbers 

 of tropical Orchidacese, etc., which are now known to be 

 regularly self-fertilising without apparent physiological injury. 

 It might also have a bearing vipon an intrusive set of facts 

 which must often have weighed upon the minds of naturalists, 

 as they reflected upon the commonly received hypothesis 

 that assumes the dangers of continued breeding between 

 near of kin. A. E. Wallace speaks of these facts in 

 " Darwinism," t and I have drawn attention to them in dis- 

 cussing the meaning of insect migration, although, as will 

 be seen in the following passage, without any serious doubt 

 as to the physiological significance of cross-fertilisation. | 



" We may well inquire why it should be necessary for such 

 emigration, with a possible successful issue in colonisation, to 

 require the services of countless individuals when the importa- 

 tion of half-a-dozen rabbits or a few specimens of Pieris rapse, 

 will, for the naturalist, change the face of a continent. The 

 resvilts of these unintentional, or intentional but ill-considered, 

 experiments do indeed shake the belief in the paramount 

 necessity for crosses and the dangers of in-and-in breeding ; 

 but the end is not yet, and the teeming colonies which have 

 arisen from such small beginnings may in time vanish from 

 the operation of deep-seated causes. The varied adaptations 

 for cross-fertilisation and the pi'evention of in-andin breeding 



* Sec also A. E. Wallace in "Danvinisin," London, 1889, pp. 321-326, 

 t p. 326. X Trans, Ent. Sec. Lonci., 1902, pp. 460-465, 



