Read it through once

There is also some reason to suppose that the antelopes and other creatures do help the plants in their efforts to colonize the Sahara. Their droppings will very greatly improve the soil, and more vigorous thickets and undergrowth will spring up when the soil is improved in this way. Such a vigorous growth of plants will be better able to resist the long eight or nine months' drought, and so help the wood to develop, until perhaps it is too thick, and the trees are too high, for the antelopes to graze upon them. In this manner the Acacia scrub is slowly and painfully colonizing the desert.