DISPERSAL OF DOMINANT RACES. 
389 
Tliese improved races as evolved become the dominant ones and 
rapidly increase in numbers, so that dispersal becomes a necessity, and 
they will therefore gradually spread and drive before them to the 
mountains, the deserts, or to more remote or inhospitable districts 
those of the preceding occupants with which they enter into the 
closest competition, and this, not necessarily by means of active 
Fig. 722. — Map illustrating the geographical continuity of distribution and the occupation of the 
primary evolutionary area by the dominant Pentatpeniate Helicidas poviatia, etc.). 
Fig. 723. — Map broadly illustrative of the distribution of the less advanced yet comparatively 
highly organized genera Helicella (of which Helix virgata may be considered as a type), Helico- 
(ionta{\\fh\c\\ embraces Helix oln>oliita) 2 .r\di Helicigona (of which Helix arbustoitivi is representa- 
tive), and shewing the progress of their expulsion from the primary evolutionary area and the 
initiation of discontinuity of distribution. 
