s
rarity would be a stage towards its extinction. Even in man, so
infinitely better known than any other inhabitant of this world, how
impossible it has been found, without statistical calculations, to judge
of the proportions of births and deaths, of the duration of life, and of
the increase and decrease of population; and still less of the causes of
such changes: and yet, as has so often been repeated, decrease in
numbers or rarity seems to be the high-road to extinction. To marvel at
the extermination of a species appears to me to be the same thing as to
know that illness is the road to death,--to look at illness as an
ordinary event, nevertheless to conclude, when the sick man dies, that
his death has been caused by some unknown and violent agency{338}.

{338} An almost identical sentence occurs in the _Origin_, Ed. i.
p. 320, vi. p. 462.

In a future part of this work we shall show that, as a general rule,
groups of allied species{339} gradually appear and disappear, one after
the other, on the face of the earth, like the individuals of the same
species: and we shall then endeavour to show the probable cause of this
remarkable fact.

{339} _Origin_, Ed. i. p. 316, vi. p. 457.

CHAPTER VI

ON THE GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS IN PAST AND PRESENT
TIMES

For convenience sake I shall divide this chapter into three
sections{340}. In the first place I shall endeavour to state the laws of
the distribution of existing beings, as far as our present object is
concerned; in the second, that of extinct; and in the third section I
shall consider how far these laws accord with the theory of allied
species having a common descent.

{340} Chapters XI and XII in the _Origin_, Ed. i., vi. chs. XII and
XIII ("On geographical distribution") show signs of having been
originally one, in the fact that one summary serves for both. The
geological element is not separately treated there, nor is there a
separate section on "how far these laws accord with the theory,
&c."

In the MS. the author has here written in the margin "If same
species appear at two spot at once, fatal to my theory." See
_Origin_, Ed. i. p. 352, vi. p. 499

SECTION FIRST.

_Distribution of the inhabitants in the different continents._

In the following discussion I shall chiefly refer to terrestrial
mammifers, inasmuch as they are better known; their differences in
diff