SUMMARY. 425 



As exemplifying the effects of climatical changes on dis- 

 tribution, I have attempted to show liow important a pai*t 

 the hist Glacial period has played, which affected even the 

 equatorial regions, and which, during the alternations of 

 the cold in the north and the south, allowed the produc- 

 tions of opposite hemispheres to mingle, and left some of 

 them stranded on the mountain-summits in all parts of the 

 world. As showing how diversified are the means of oc- 

 casional transport, I have discussed at some little length 

 the means of dispersal of fresh-water productions. 



If the difficulties be not insuperable in admitting that 

 in the long course of time all tiie individuals of the 

 same species, and likewise of the several species belonging 

 to the same genus, have proceeded from some one source; 

 then all the grand leading facts of geographical distribution 

 are explicable on the theory of migration, together with 

 subsequent modification and the multiplication of new 

 forms. We can thus understand the high imjoortance of 

 barriers, whether of land or water, in not only separating 

 but in apparently forming the several zoological and 

 botanical provinces. We can thus understand the con- 

 centration of related species within the same areas; and 

 how it is that under different latitudes, for instance, in 

 South America, the inhabitants of the plains and mount- 

 ains, of the forests, marshes and deserts, are linked 

 together in so mysterious a manner, and are likewise 

 linked to the extinct beings which formerly inliabited 

 the same continent. Bearing in mind that the mutual 

 relation of organism to organism is of the highest im- 

 portance, we can see why two areas, having nearly the 

 same physical conditions, should often be inhabited by 

 very different forms of life; for according to the length 

 of time which has elapsed since the colonists entered one 

 of the regions, or both; according to the nature of the 

 communication which allowed certain forms and not others 

 to enter, either in greater or lesser numbers; according or 

 not as those which entered happened to come into more 

 or less direct competition with each other and with the 

 aborigines; and according as the immigrants were capable 

 of varying more or less rapidly, there would ensue in the 

 two or more regions, independently of their physical con- 

 ditions, infinitely diversified conditions of life; there would 



