increased, and yet if it continues to
increase ever so slowly (without the fertility of the species in
question be likewise increased) the average number of the individuals of
that species must decrease, and become finally lost. I may give a single
instance of a check causing local extermination which might long have
escaped discovery{336}; the horse, though swarming in a wild state in La
Plata, and likewise under apparently the most unfavourable conditions in
the scorched and alternately flooded plains of Caraccas, will not in a
wild state extend beyond a certain degree of latitude into the
intermediate country of Paraguay; this is owing to a certain fly
depositing its eggs on the navels of the foals: as, however, man with a
_little_ care can rear horses in a tame state _abundantly_ in Paraguay,
the problem of its extinction is probably complicated by the greater
exposure of the wild horse to occasional famine from the droughts, to
the attacks of the jaguar and other such evils. In the Falkland Islands
the check to the _increase_ of the wild horse is said to be loss of the
sucking foals{337}, from the stallions compelling the mares to travel
across bogs and rocks in search of food: if the pasture on these islands
decreased a little, the horse, perhaps, would cease to exist in a wild
state, not from the absolute want of food, but from the impatience of
the stallions urging the mares to travel whilst the foals were too
young.

{333} This corresponds approximately to _Origin_, Ed. i. p. 317,
vi. p. 458.

{334} The case of _Trigonia_, a great Secondary genus of shells
surviving in a single species in the Australian seas, is given as
an example in the _Origin_, Ed. i. p. 321, vi. p. 463.

{335} This point, on which the author laid much stress, is
discussed in the _Origin_, Ed. i. p. 319, vi. p. 461.

{336} _Origin_, Ed. i. p. 72, vi. p. 89.

{337} This case does not occur in the _Origin_, Ed.

From our more intimate acquaintance with domestic animals, we cannot
conceive their extinction without some glaring agency; we forget that
they would undoubtedly in a state of nature (where other animals are
ready to fill up their place) be acted on in some part of their lives by
a destroying agency, keeping their numbers on an average constant. If
the common ox was known only as a wild S. African species, we should
feel no surprise at hearing that it was a very rare species; and thi