Alph. de Candolle has shown that
those species which spread widely tend generally to spread VERY widely,
consequently they will tend to supplant and exterminate several species in
several areas, and thus check the inordinate increase of specific forms
throughout the world. Dr. Hooker has recently shown that in the southeast
corner of Australia, where, apparently, there are many invaders from
different quarters of the globe, the endemic Australian species have been
greatly reduced in number. How much weight to attribute to these several
considerations I will not pretend to say; but conjointly they must limit in
each country the tendency to an indefinite augmentation of specific forms.
SUMMARY OF CHAPTER.
If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions of
life, causing an infinite diversity in structure, constitution, and habits,
to be advantageous to them, it would be a most extraordinary fact if no
variations had ever occurred useful to each being's own welfare, in the
same manner as so many variations have occurred useful to man.
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