Cees) 
unceasing selection. Comparatively brief isolation of a group 
of individuals may lead to a departure from the specific type 
of apparatus prevalent in other areas, and may thus mechanic- 
ally prevent syngamy if from any cause members of the 
group became again sympatric with those of the parent 
species. 
A very different but exceedingly interesting origin of 
asyngamy is suggested by observations which support the 
conclusion that varietal forms may show a tendency towards 
preferential inter-breeding. 
H. W. Bates believed that he had strong evidence for the 
existence of this tendency in the races of certain tropical 
American butterflies. He stated this in his epoch-making 
paper on the butterflies of the Amazon valley,* and it is 
interesting to observe in the published letters how Darwin 
instantly fixed upon the point and tried to elicit the data 
upon which the conclusion was formed. Thus he wrote to 
Bates, Nov. 20 [1862]:—“ No doubt with most people this 
[viz. the interpretation of Mimicry] will be the cream of the 
paper; but I am not sure that all your facts and reasonings 
on variation, and on the segregation of complete and semi- 
complete species, is not really more, or at least as valuable, a 
part. I never conceived the process nearly so clearly before ; 
one feels present at the creation of new forms. I wish, 
however, you had enlarged a little more on the pairing of 
similar varieties ; a rather more numerous body of facts seems 
here wanted.” fT 
Then a few days later we find Darwin still thinking of the 
subject, and writing to Hooker [1862, Nov.] 24:—‘I have 
now finished his [Bates’] paper . . .; it seems to me admir- 
able. To my mind the act of segregation of varieties into 
species was never so plainly brought forward, and there are 
heaps of capital miscellaneous observations.” { 
He also again wrote to Bates, probably on the following 
day, Nov. 25 [1862 1%], asking for the solid facts which are so 
greatly wanted :— 
“Could you find me some place, even a footnote (though 
* Trans. Linn. Soe., vol. xxiii (1862), p. 495. 
+ ‘‘ Life and Letters,” vol. ii, p. 392. 
£ ‘*More Letters,” vol. i, p. 214, Letter 147. 
