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for the conversion of its offspring, firstly into new varieties and ultimately into new species. ↑
In considering the wide distribution of certain genera, we should bear in mind that some are extremely ancient, and that the species must have branched off from a common parent at a remote epoch; so that in such cases there will have been ample time for great climatal and geographical changes and for accidents of transport; and consequently for the migration of some of the species into all quarters of the world, where they may have become slightly modified in relation to their new conditions. There is, also, some reason to believe from geological evidence that organisms low in the scale within each great class, generally change at a slower rate than the higher forms; and consequently the lower forms will have had a better chance of ranging widely and of still retaining the same specific character. This fact, together with the seeds and eggs of many low forms being very minute and better fitted for distant transportation, probably accounts for a law which has long been observed, and which has lately been admirably discussed by Alph. de Candolle in regard to plants, namely, that the lower any group of organisms is, the more widely it is apt to range.
The relations just discussed,— namely, low and slowly-changing organisms ranging more widely than the high,— some of the species of widely-ranging genera themselves ranging widely,— such facts, as alpine, lacustrine, and marsh productions being related (with the exceptions before specified) to those on the surrounding low lands and dry lands, though these stations are so different,— the very close relation of the distinct species which inhabit the islets of the same archipelago,— and especially the striking relation of the inhabitants of each whole archipelago or island to those