The three lowest orders of mammals, namely, marsupials,
edentata, and rodents, co-exist in South America in the same region with
numerous monkeys, and probably interfere little with each other. Although
organisation, on the whole, may have advanced and be still advancing
throughout the world, yet the scale will always present many degrees of
perfection; for the high advancement of certain whole classes, or of
certain members of each class, does not at all necessarily lead to the
extinction of those groups with which they do not enter into close
competition. In some cases, as we shall hereafter see, lowly organised
forms appear to have been preserved to the present day, from inhabiting
confined or peculiar stations, where they have been subjected to less
severe competition, and where their scanty numbers have retarded the chance
of favourable variations arising.
Finally, I believe that many lowly organised forms now exist throughout the
world, from various causes. In some cases variations or individual
differences of a favourable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few cases
there has been what we must call retrogression or organisation. But the
main cause lies in the fact that under very simple conditions of life a
high organisation would be of no service--possibly would be of actual
disservice, as being of a more delicate nature, and more liable to be put
out of order and injured.
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