XVI PREFACE. the green colouring matter of plants, which can only be developed by the presence of light. The tops of celery, being unearthed, retain their green colour, while the stem embedded in the soil acquires its familiar whiteness. Many philosophical writers, notably David Hume and Charles Comte, C. Montesquieu in his L'Esprit des Lois, and Henry Thomas Buckle in his History of Civilization in England, have dilated upon the influence which climate exerts over race, and all their forceful opinions are to the effect that the character of a people is moulded by climatic con- ditions. More than this, the same view was enter- tained by the classic writers ; for we find the philo- sopher and orator Cicero recording his belief that. " Athens has a light atmosphere, whence the Athenians " are thought to be more keenly intelligent ; Thebes " a dense one, and the Thebans fat-witted accord- " ingly." Again, Horace, the poet and satirist, has given us the famous passage : — " You would swear he " (Alexander the Great) was born in the dense atmo- " sphere of the Boeotians." But the influence of climate is not confined to ordinary conditions alone, because without the shadow of a doubt it controls disease as well. As it is well known, certain diseases are peculiar to, and confined to, certain regions. And, moreover, a malady will vary in its type in different zones. Thus the disease
