Origin of Species Variorum

Comparison with 1869

invertebrate classes. But mammals and fish hardly come into competition with each other; the advancement of the whole class of mammals, or of certain members in this class, to the highest grade .. would not lead to their taking the place of, and thus exterminating, fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aërial respiration; so that warm-blooded mammals when inhabiting the water live under some disadvantages in comparison with fishes. ↑ In this latter class, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Müller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid. 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

invertebrate classes. But mammals and fish hardly come into competition with each other; the advancement of certain mammals or of the whole class to the highest grade of organisation would not lead to their taking the place of, and thus exterminating, fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aërial respiration; so that warm-blooded mammals when inhabiting the water live under some disadvantages compared with fishes. In this latter class, members of the shark family would not, it is probable, tend to supplant the lancelet; the struggle for existence in the case of the lancelet apparently will lie with members of the invertebrate classes. ↑ 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 advancing throughout the world, yet the scale will still present all 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 seem to have been preserved to the present day from inhabiting peculiar or isolated stations, where they have been subjected to less severe competition, and where they have existed in scanty numbers, which, as already explained, retards the chance of favourable variations arising.

Finally, I believe that lowly organised forms now exist in numbers throughout the world, and in nearly every class, from various causes. In some cases favourable variations may never have arisen for natural selec- tion