398 SUMMARY. 



SUMMARY OF THE LAST AND PRESENT CHAPTERS. 



In these chapters I have endeavored to show that if we 

 .make due allowance for our ignorance of the full effects of 

 changes of climate and of the level of the land, which have 

 certainly occurred within the recent period, and of other 

 changes which have probably occurred — if we remember 

 .how ignorant we are with respect to the many curious 

 means of occasional transport — if we bear in mind, and this 

 is a very important consideration, how often a species may 

 have ranged continuously over a wide area, and then have 

 .become extinct in the intermediate tracts — the difficulty is 

 not insuperable in believing that all the individuals of the 

 same species, wherever found, are descended from common 

 .parents. And we are led to this conclusion, which has been 

 arrived at by many naturalists under the designation of 

 single centres of creation, by various general considerations, 

 more especially from the importance of barriers of all kinds, 

 and from the analogical distribution of subgenera, genera, 

 and families. 



With respect to distinct species belonging to the same 

 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 indi- 

 viduals of the same species, they are often great. 

 , As exemplifying the effects of climatical changes on dis- 

 tribution, 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 the 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 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 j then all 



