THE MULTIPLICATION OF EFFECTS. 457 



change of food, as well as by change of climate; and the 

 modification would be more marked where, from the dwin- 

 dling or disappearance of one kind of plant, an allied kind 

 was eaten. In the lapse of the many generations arising be- 

 fore the next upheaval, the sensible or insensible alterations 

 thus produced in each species, would become organized 

 in all the races that survived there would be a more or less 

 complete adaptation to the new conditions. The next up- 

 heaval would superinduce further organic changes, imply- 

 ing wider divergences from the primary forms; and so 

 repeatedly. Now however let it be observed that this revo- 

 lution would not be a substitution of a thousand modified 

 species for the thousand original species ; but in place of the 

 thousand original species there would arise several thousand 

 species, or varieties, or changed forms. Each species being 

 distributed over an area of some extent, and tending con- 

 tinually to colonize the new area exposed, its different mem- 

 bers would be subject to different sets of changes. Plants 

 and animals migrating towards the equator would not be 

 affected in the same way with others migrating from it. 

 Those which spread towards the new shores, would undergo 

 changes unlike the changes undergone by those which 

 spread into the mountains. Thus, each original race of or- 

 ganisms would become the root from which diverged sev- 

 eral races, differing more or less from it and from each 

 other; and while some of these might subsequently disap- 

 pear, probably more than one would survive in the next 

 geologic period: the very dispersion itself increasing the 

 chances of survival. Not only would there be certain modi- 

 fications thus caused by changes of physical conditions and 

 food; but also in some cases other modifications caused by 

 changes of habit do take place in animals; and we know 

 by step, the newly-raised tracts, would eventually come in 

 contact with the faunas of other islands ; and some members 

 of these other faunas would be unlike any creatures before 

 seen. Herbivores meeting with new beasts of prey, would, 

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