Read it through once

Of this there cannot be any reasonable doubt, for the extraordinary quantity of thorny, spiny things in deserts shows that there must be some connexion between such a life and their production (see Chapter X.). In such places animals are always abundant. But these hollies, pears, and other plants show exactly the opposite to what we should expect. It is when the head of the young holly reaches the sunlight and feels the wind that its leaves become harmless!