( 47 ) 
these injurious results have been not_ the cause but the con 
sequence of specialisation for cross-fertilisation. In such 
plants fertil sation 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 results 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 numbers 
of tropical Orchidacez, etc., which are now known to be 
regularly self-fertilising without apparent physiological injury. 
It might also have a bearing upon 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. R. Wallace speaks of these facts in 
“ Darwinism,” + 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 rapx 
will, for the naturalist, change the face of a continent. The 
results 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 prevention of in-and-in breeding 
* See also A. R. Wallace in ‘‘ Darwinism,” London, 1889, pp. 321-326. 
+ p. 326. ~ Trans. Ent. Soc. Lond., 1902, pp. 460-465. 
