THE BREAKDOWN OF CONTINUITY 



on its possessors and so will increase in frequency. The environmental 

 isolation will be reinforced by genetic means. And as the restriction 

 on crossing becomes greater, the relational balance will get even 

 worse. Hybrid incapacity will increase, and with it the advantage of 

 gcnctical bars to crossing. The system must therefore be self- 

 stimulating, and once started, isolation will sooner or later become 

 complete. Furthermore, the bars to crossing which are thus 

 stimulated indirectly by loss of relational balance, may be 

 reinforced by the direct expression of differences in form and 

 function which arise as expressions of the increasing divergence 

 of the two groups (Fig. 77). 



Where divergence has followed the course just described, the 

 bars to crossing will generally prevent, or at least make difficult, 

 crossing by artificial means. This is so, for example, witli the 

 AtitirrJiwwn species, tnajtis and orontinm. One of the bars to crossing 

 between them is the prevention of growth in the style of one 

 species, of pollen from the other. Such a bar operates against 

 artificial as well as natural cross-pollination. The two species, majus 

 and glutinoswn, however, show us that the bar to natural crossing 

 need not prevent artificial hybridization. When grown together and 

 allowed to pollinate naturally under English conditions, they show 

 less than 3 per cent crossing. They can, nevertheless, be crossed 

 readily by artificial means. There appears to be no barrier to cross- 

 fertilization once cross-pollination has been achieved. Flere the 

 isolating mechanism is found to consist in the failure of cross- 

 pollination, consequent on the failure of pollinating insects, mainly 

 bees, to visit flowers of the two species alternately when making 

 their working flights on a mixed stand of plants. 



In these species o£ Antirrhinum the plants have, so to speak, taken 

 advantage of the insects' power to discriminate between different 

 flowers, and of their predilection for confining their visits on one 

 run to flowers of the same type. We have, therefore, the remarkable, 

 but possibly frequent, situation of isolation between two species 

 depending on the habits of a third. Flowers are, of course, generally 

 adapted in form and colour to the insect which pollinates them. 

 Here, in turn, not only the form but the discontinuity in form of 

 the flower depends on the habits of the insect. 



308 



