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.