GEOGRAPHICAL DISTRIBUTION 367 



case of certain powerfully winged birds, will necessarily range 

 widely; for we should never forget that to range widely implies 

 not only the power of crossing barriers, but the more important 

 power of being victorious in distant lands in the struggle for life 

 with foreign associates. But according to the view that all the 

 species of a genus, though distributed to the most remote points 

 of the world, are descended from a single progenitor, we ought 

 to find, and I believe as a general rule we do find, that some at 

 least of the species range very widely. 



We should bear in mind that many genera in all classes are of 

 ancient origin, and the species in this case will have had ample 

 time for dispersal and subsequent modification. There is also 

 reason to believe, from geological evidence, that within each 

 great class the lower organisms change at a slower rate than the 

 higher; consequently they will have had a better chance of rang- 

 ing widely and of still retaining the same specific character. This 

 fact, together with that of the seeds and eggs of most lowly or- 

 ganized forms being very minute and better fitted for distant 

 transportal, probably accounts for a law which has long been ob- 

 served, and which has lately been discussed by Alph. de CandoUe, 

 in regard to plants; namely, that the lower any group of organ- 

 isms stands, the more widely it ranges. 



The relations just discussed — namely, lower organisms rang- 

 ing more widely than the higher — ^some of the species of widely 

 ranging genera themselves ranging widely — such facts, as alpine, 

 lacustrine, and marsh productions being generally related to those 

 which live on the surrounding low lands and dry lands — the 

 striking relationship between the inhabitants of islands and those 

 of the nearest mainland — the still closer relationship of the dis- 

 tinct inhabitants of the islands in the same archipelago — are in- 

 explicable on the ordinary view of the independent creation of 

 each species, but are explicable if we admit colonization from the 

 nearest or readiest source, together with the subsequent adapta- 

 tion of the colonists to their new homes. 



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 oc- 

 curred 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 



