( xvii ) 



direction. As long as a group of individuals is syngamic (i. e. 

 is a species) the genitalia may vary similarly throughout the 

 group, but may not run into definite varieties. When, how- 

 ever, such a group is divided into two portions or groups by 

 geographical seasonal or other segregation, each portion must 

 keep a form of appendages constant throughout the portion, 

 but probably drifting away from those of the other group, 

 which may be varying in another direction. When such 

 drifting has proceeded far enough, the two portions are no 

 longer syngamic, and act towards each other as distinct species, 

 should opportunity of mixing occur. It seems very probable 

 that such drifting apart may take place with considerable 

 rapidity, in view of the many cases we know, of two or 

 three species quite distinct as to the genitalia, quite asyngamic, 

 and yet in external character much alike, even difficult to dis- 

 tinguish. It is very probable that such pairs of species whilst 

 segregated may have drifted apart in external characters, but 

 when subjected again to the same environment come to 

 resemble one another very closely, quite apart from any 

 question of mimicry. 



" The dorsal armature of the genitalia of argus and argyro- 

 gnomon seems to me to negative such an hypothesis, but it is 

 really nevertheless possible, and is indeed the hypothesis 

 commonly held, that these two are species derived from a 

 common stock in some such way as I have indicated. 



" The point I wish to direct attention to as bearing on such 

 cases, is that if a comparatively small portion of a species be 

 fully segregated, it may vary more or less in external charac- 

 ters taking up a position suitable to the new and restricted 

 environment and there remaining constant, but that the 

 genitalia are under no restrictions, being free to vary to any 

 extent, subject only to all within the group varying together. 

 When the group is small in numbers, a tendency to vary will 

 have greater possibility of resulting in change, as there will 

 be no large mass of individuals to absorb contrary tendencies 

 and by its inertia prevent change. Whilst, therefore, little 

 or no change in the genitalia may take place in some or a 

 majority of cases, in others it will probably take place with 

 some rapidity and to a considerable extent. 



PROC. ENT. 80C. LOND., III. 1909. B 



