360 Summary of the Chap. xiii. 



enormous periods of time having been thus granted for their 

 migration, the difficulties are far from insuperable; though in 

 this case, as in that of the individuals of the same species, they 

 are often great. 



As exemplifying the effects of climatal changes on distribution 

 I have attempted to show how important a part the last Glacial 

 period has played, which affected even the equatorial regions 

 and which, during the alternations of the cold in the north and 

 south, allowed the productions of opposite hemispheres to mino-i e 

 and left some of them stranded on the mountain- summits in all 

 parts of the world. As showing how diversified are the means 

 of occasional 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 the 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 multipli- 

 cation of new forms. We can thus understand the high importance 

 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 concentration 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 

 mountains, of the forests, marshes, and deserts, are linked together 

 in so mysterious a manner, and are likewise linked to the extinct 

 beings which formerly inhabited the same continent. Bearing in 

 mind that the mutual relation of organism to organism is of the 

 highest importance, 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 ; accord- 

 ing 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, inde- 

 pendently of their physical conditions, infinitely diversified con- 

 ditions of life, — there would be an almost endless amount of organic 

 action and reaction, — and we should find some groups of beings 

 greatly, and some only slightly modified,— some developed in great 



