Origin of Species Variorum

for the variability of each species is quite independent of that of all others. Whether such variations or individual differences as may arise will be accumulated through natural selection in a greater or less degree, thus causing a greater or less amount of permanent modification, will depend on many complex contingencies— on the variations being of a beneficial nature, on the freedom of intercrossing, on the ... slowly changing physical conditions of the country, ... on the immigration of new colonists, and on the nature of the other inhabitants with which the varying species come into competition. Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change in a less degree. We find similar relations between the inhabitants of distinct countries; for instance, .. the land-shells and coleopterous insects of Madeira have come to differ considerably from their nearest allies on the continent of Europe, whereas the marine shells and birds have remained unaltered. We can perhaps understand the apparently quicker rate of change in terrestrial and in more highly organised productions compared with marine and lower productions, by the more complex relations of the higher beings to their organic and inorganic conditions of life, as explained in a former chapter. When many of the inhabitants of any area have become modified and improved, we can understand, on the principle of competition, and from the all-important relations of organism to organism in the struggle for life, that any form which does not become in some degree modified and improved, will be liable to .. extermination. Hence we .. see why all the species in the same region do at last, if we look to long enough intervals of time, become modified, for otherwise they would