At the same time it should be remembered that we know little of the climatic conditions at the time of the first migrations, though it has been assumed that it was everywhere much milder than at present. Consequently the different zones of temperature were less marked, and the passage from one region to another more easily effected than in later times. In a word the pleistocene precursors had far less difficulty in adapting themselves to their new surroundings than modern peoples have when they emigrate, for instance, from Southern Europe to Brazil and Paraguay, or from the British Isles to Rhodesia and Nyassaland. What is true of man must be no less true of his works; from which it follows that racial and cultural zones correspond in the main with zones of temperature, except so far as the latter may be modified by altitude, marine influences, or other local conditions. A glance at past and existing relations the world over will show that such harmonies have at all times prevailed. No doubt the overflow of the leading European peoples during the last 400 years has brought about divers dislocations, blurrings, and in places even total effacements of the old landmarks. But, putting aside these disturbances, it will be found that in the Eastern hemisphere the inter-tropical regions, hot, moist and more favourable to vegetable than to animal vitality, are usually occupied by savage, cultureless populations. Within the same sphere are also comprised most of the extra-tropical southern lands, all tapering towards the antarctic waters, isolated, and otherwise unsuitable for areas of higher specialisation. Similarly the sub-tropical Asiatic peninsulas, the bleak Tibetan tableland, the Pamir, and arid Mongolian steppes are found mainly in possession of somewhat stationary communities, which present every stage between sheer savagery and civilisation. In the same way the higher races and cultures are confined to the more favoured north temperate zone, so that between the parallels of 24 deg. and 50 deg. (but owing to local conditions falling in the far East to 40 deg. and under, and in the extreme West rising to 55 deg.) are situated nearly all the great centres, past and present, of human activities--the Egyptian, Babylonian, Minoan (Aegean), Hellenic, Etruscan, Roman, and modern European. Almost the only exceptions are the early civilisations (Himyaritic) of Yemen (Arabia Felix) and Abyssinia, where the low latitude is neutralised by altitude and a copious rainfall. Thanks also to altitude, to marine influences, and the contraction of the equatorial lands, the relations are almost completely reversed in the New World. Here all the higher developments took place, not in the temperate but in the tropical zone, within which lay the seats of the Peruvian, Chimu, Chibcha and Maya-Quiche cultures; the Aztec sphere alone ranged northwards a little beyond the Tropic of Cancer.