THE BASIS OF ADAPTATION 261 



The agency of other organisms is probably of greater importance 

 than we reahze in dispersal. The transportation of plant seeds 

 by birds and other animals is familiar. It may come about through 

 use of fruits as food and failure to digest the seeds, or through 

 accidental means. The transportation of such specialized fruits 

 as the cockleburs, beggar's-ticks and Spanish needles by animals 

 is furthered by their special adaptations for adhesion to fur or 

 skin. Small animals may also be transported to some degree in 

 this way. Parasitic, commensal and symbiotic species are so 

 distributed, beyond doubt, but the activities of men are probably 

 of greater importance in the dispersal of free living animals than 

 the incidental associations of lower forms. The railroads and 

 other vehicular traffic are known to have carried organisms 

 accidentally for long distances, and some of our most troublesome 

 pests, the San Jose scale, gypsy moth, Japanese beetle, European 

 corn borer, English sparrow and starling have been imported, 

 intentionally or accidentally, from other continents. 



Agencies of this kind may well transport animals from one land 

 mass to another accidentally, but the results would be similar to 

 those arising from forced migration. Accidental transportation, 

 however, would be much less likely to carry organisms into regions 

 favorable for their development than intentional migration. 



For these reasons no part of the world is able to remain long 

 without a population of living organisms if it provides the bare 

 necessities of life. Sooner or later, through accident or the pressure 

 of competition in other regions it receives pioneers which may or 

 may not be able to maintain themselves. The repetition of such 

 occurrences is bound in time to result in the establishment of 

 some species and with every addition the possibility of adaptive 

 branching increases. Ultimately the region is itself a source of 

 migrants to other places. 



The Constancy of Change. It must not be supposed that 

 these are abrupt transitions. Only such limited land masses as coral 

 and volcanic islands, lava flows, and inundated and glaciated re- 

 gions can have been utterly devoid of life at any time since life be- 

 gan. The transitions of most of these have been gradual, and with 

 their gradual changers must have come gradual development of 

 flora and fauna as opportunity arose. 



In most regions life has been continuous, as it is now, and as 

 now, fluctuating in details. Emigration and immigration go on 



