INTROGRESSION AND EVOLUTION 63 



Like man himself all these phenomena alter conditions 

 catastrophically, break down barriers between species, and 

 provide miusual new habitats in which hybrid derivatives 

 may for a time find a foothold, thus serving as a bridge by 

 which groups of genes from one species can invade the germ- 

 plasm of another. 



Not until one has lived in close proximity to a large mid- 

 continental river does he realize what a restless neighbor 

 such a waterway can be. It is forever changing its course 

 and altering the habitats of plants that grow near it. Trees 

 are undermined and swept away; sand to the depth of sev- 

 eral feet is deposited on top of heavy clays or silt, thus 

 changing the soil type and the ground-water level; plants 

 are transported bodily; and not only do water levels change 

 from day to day and week to week, but also the average 

 level of the previous decade may be drastically altered by a 

 whim of the river. In such a variable environment species 

 that (through introgression) are able to achieve a great in- 

 crease in genie variability should be at a selective advantage. 

 It is apparently true that river-valley plants are more gen- 

 erally adaptable than those from other habitats. It would 

 seem likely that introgression may be one of the natural 

 forces that have brought about this greater adaptability. 

 Exact data bearing on this point should not be difficult to 

 obtain. 



A demonstration of the evolutionary importance of ' ^nat- 

 ural' ^ introgression on a much wider scale is emerging from 

 a series of studies by various workers which are already well 

 under way but for the most part have not yet been formally 

 published. All suggest the probable importance of intro- 

 gression at particular times and places when diverse floras 

 were brought together in a changing environment. Mason 

 and his collaborators (1942; see also Cain, 1944), working 

 with living and fossil populations of the closed cone pines, 

 are finding it possible to demonstrate these phenomena in a 

 surprisingly exact fashion. Areas that were once a series of 

 islands off the California coast have been united to the main- 



