The entire variation from the original type, which any given kind
of change can produce, may usually be effected in a brief period of
time, after which no farther deviation can be obtained by continuing to
alter the circumstances, though ever so gradually; indefinite
divergence, either in the way of improvement or deterioration, being
prevented, and the least possible excess beyond the defined limits being
fatal to the existence of the individual.

5thly. The intermixture of distinct species is guarded against by the
aversion of the individuals composing them to sexual union, or by the
sterility of the mule offspring. It does not appear that true hybrid
races have ever been perpetuated for several generations, even by the
assistance of man; for the cases usually cited relate to the crossing of
mules with individuals of pure species, and not to the intermixture of
hybrid with hybrid.

6thly. From the above considerations, it appears that species have a
real existence in nature; and that each was endowed, at the time of its
creation, with the attributes and organization by which it is now
distinguished.

CHAPTER XXXVII.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF SPECIES.

Analogy of climate not attended with identity of species--Botanical
geography--Stations--Habitations--Distinct provinces of indigenous
plants--Vegetation of islands--Marine vegetation--In what manner
plants become diffused--Effects of wind, rivers, marine
currents--Agency of animals--Many seeds pass through the stomachs of
animals and birds undigested--Agency of man in the dispersion of
plants, both voluntary and involuntary--Its analogy to that of the
inferior animals.

Next to determining the question whether species have a real existence,
the consideration of the laws which regulate their geographical
distribution is a subject of primary importance to the geologist. It is
only by studying these laws with attention, by observing the positions
which groups of species occupy at present, and inquiring how these may
be varied in the course of time by migrations, by changes in physical
geography, and other causes, that we can hope to learn whether the
duration of species be limited, or in what manner the state of the
animate world is affected by the endless vicissitudes of the inanimate.

_Different regions inhabited by distinct species._--That different
regions of the globe are inhabited by entire