VARIOUS CAUSES OF ASVXGAMY 85 



of Pyrameis cardui (the ' Painted Lady ' butterfly), which 

 ranges over nearly the whole world. The singular rarity 

 of local geographical races l in this abundant species 

 is almost certainly due to the astonishing powers of dis- 

 persal which enable intermittent Syngamy to prevail over 

 the nearly whole vast area of its distribution. 



Asyngamy as a consequence of Mechanical Incompati- 

 bility. 



An interesting and curious cause of persistent Asyngamy 

 is the ' Mechanical Selection ' so thoroughly explained 

 and abundantly illustrated by Karl Jordan.- The com- 

 plex genital armature of Lepidoptera is during Syngamy 

 kept constant by 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 mechanically prevent Syn- 

 gamy if from any cause members of the group became 

 again sympatric with those of the parent species. 



Asyngamy as a consequence of Prefei-ential Mating. 



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 interbreeding. 



H. \Y. 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, 3 and it is interesting to observe in the published 



material in recent years has revealed peculiarities in the Madeiran birds 

 which were unknown when Darwin wrote. But the general argument 

 still holds good, even though our knowledge has been increased and 

 modified. Madeiran birds have changed but slightly as compared with 

 those of the Galapagos Islands, where interbreeding with the related 

 mainland species has been almost or entirely prevented by local con- 

 ditions contrasting sharply with those by which Madeira is surrounded. 



1 There is only the small, slightly modified form, kershawt, from the 

 Australian Region. 



- Norilales Zoologicae, vol. iii. Dec. 1896. pp. 518-22. 



3 Trans. Linn. Soc. vol. xxiii (1862), p. 495. 



