but becoming more important after the age of Reptiles, to which we shall proceed in our next chapter. These are the effects produced upon climate by life itself. Particularly great is the influence of vegetation, and especially that of forests. Every tree is continually transpiring water vapour into the air; the amount of water evaporated in summer by a lake surface is far less than the amount evaporated by the same area of beech forest. As in the later Mesozoic and the Cainozoic Age, great forests spread over the world, their action in keeping the air moist and mitigating and stabilizing climate by keeping the summer cool and the winter mild must have become more and more important. Moreover, forests accumulate and protect soil and so prepare the possibility of agricultural life.
Water-weeds again may accumulate to choke and deflect rivers, flood and convert great areas into marshes, and so lead to the destruction of forests or the replacement of grass-lands by boggy wildernesses.
Finally, with the appearance of human communities, came what is perhaps the most powerful of all living influences upon climate. By fire and plough and axe man alters his world. By destroying forests and by irrigation man has already affected the climate of great regions of the world's surface. The destruction of forests makes the seasons more extreme; this has happened, for instance, in the northeastern states of the United States of America. Moreover, the soil is no longer protected from the scour of rain, and is washed away, leaving only barren rock beneath. This has happened in Spain and Dalmatia and, some thousands of years earlier, in South Arabia. By irrigation, on the other hand, man restores the desert to life and mitigates climate. This process is going on in Northwest India and Australia. In the future, by making such operations world-wide and systematic, man may be able to control climate to an extent at which as yet we can only guess.