Page - 125 - in The Origin of Species

DIVERGENCE OF CHARACTER 125 (supposing its nature not to be in any way peculiar), and may be said to be striving to the utmost to live there ; but, it is seen, that where they come into the closest competition, the advantages of diversification of structure, with the accom- panying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. The same principle is seen in the naturalisation of plants through man's agency in foreign lands. It might have been expected that the plants which would succeed in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country. It might also, perhaps, have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. But the case is very different; and Alph. de Candolle has well re- marked, in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species. To give a single instance: in the last edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United States,' 260 naturalised plants are enumerated, and these be- long to 162 genera. We thus see that these naturalised plants are of a highly diversified nature. They differ, moreover, to a large extent, from the indigenes, for out of the 162 natural- ised genera, no less than 100 genera are not there indigenous, and thus a large proportional addition is made to the genera now living in the United States. By considering the nature of the plants or animals which have in any country struggled successfully with the indigenes, and have there become naturalised, we may gain some crude idea in what manner some of the natives would have to be modified, in order to gain an advantage over their com- patriots; and we may at least infer that diversification of structure, amounting to new generic differences, would be profitable to them. The advantage of diversification of structure in the in- habitants of the same region is, in fact, the same as that of