Isolating Mechanisms 



521 



tat can be found if a swamp drainage bayou (which is never of 

 deltaic origin) cuts across one of the long-estabUshed deltaic 

 ridges. It is in an intermediate situation thus formed that almost 

 all the large mass of hybrid types is found (Fig. 148). 



Fig. 148. A map of Iris colonies illustrating an ecological barrier. At 

 F is a. group of clones of Iris fulva growing on the bank of the alluvial 

 ridge of a former deltaic stream, S, now filled in. A new bayou, B, has cut 

 across the old stream bed and at their intersection a marsh has formed in 

 which are growing clones of /. hexagona var. giganticaerulea, G. Nor- 

 mall3% the two species do not come close enough together to hj'bridize. 

 However, in a region such as this one, if man disturbs the land of the 

 old deltaic stream as at H, hybrids may be found. The pasturing of cattle 

 appears to aid in eliminating the barrier by fertilizing the soil, thinning 

 out the competitors of Iris, and keeping the soil well cultivated. Plants or 

 seeds of giganticaerulea washed up the old stream bed from G might suc- 

 cessfully become established at H, where they would meet plants of /. 

 julvdi and form a hybrid swarm. (Redrawn from Riley in the American 

 Journal of Botany.) 



Seasonal Isolation. An intrinsic isolating mechanism that pro- 

 duces the same result as ecological isolation but operates through 

 a time factor rather than through a space factor is seasonal isola- 

 tion. If two species inhabit the same geographical and ecological 

 regions but if each one is reproductively active at a time when 

 the reproductive processes of the other are dormant, they might 

 as well be thousands of miles apart for they cannot come to- 

 gether to produce hybrids. A number of species are separated 

 by this type of barrier. 



