Page - 399 - in The Origin of Species

GEOGRAPHICAL DISTRIBUTION 399 of seizing on new places, when they spread into new coun- tries. In their new homes they will be exposed to new con- ditions, and will frequently undergo further modification and improvement; and thus they will become still further vic- torious, and will produce groups of modified descendants. Or. this principle of inheritance with modification we can understand how it is that sections of genera, whole genera, and even families, are confined to the same areas, as is so commonly and notoriously the case. There is no evidence, as was remarked in the last chapter, of the existence of any law of necessary development. As the variability of each species is an independent property, and will be taken advantage of by natural selection, only so far as it profits each individual in its complex struggle for life, so the amount of modification in different species will be no uniform quantity. If a number of species, after hav- ing long competed with each other in their old home, were to migrate in a body into a new and afterwards isolated country, they would be little liable to modification ; for neither migration nor isolation in themselves effect anything. These principles come into play only by bringing organisms into new relations with each other and in a lesser degree with the surrounding physical conditions. As we have seen in the last chapter that some forms have retained nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly or at all modified. According to these views, it is obvious that the several species of the same genus, though inhabiting the most dis- tant quarters of the world, must originally have proceeded from the same source, as they are descended from the same progenitor. In the case of those species which have under- gone during whole geological periods little modification, there is not much difficulty in believing that they have mi- grated from the same region ; for during the vast geographi- cal and climatal changes which have supervened since ancient times, almost any amount of migration is possible. But in many other cases, in which we have reason to believe that the species of a genus have been produced within compara- tively recent times, there is great difficulty on this head. It