SUMMARY 447 



genus, which on our theory have spread from one parent- 

 source ; if we make the same allowances as before for our 

 ignorance, and remember that some forms of life have 

 changed very slowly, 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 distribu- 

 tion. I have attempted to show how important a part the last 

 Glacial period has played, which affected even the equa- 

 torial regions, and which, during the alternations of the cold 

 in the north and south, allowed the productions 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 occasional transport, I have dis- 

 cussed 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 explic- 

 able on the theory of migration, together with subsequent 

 modification and the multiplication 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 condi- 

 tions 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; ac- 

 cording to the nature of the communication which allowed 



