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We appear to take it as a rule, or as a law of nature, that each species is adapted to the climate of its own home. For example, species from the Arctic, or even a temperate region, could not survive in a tropical climate, nor could a tropical species last long if it found itself at the South Pole. But it is true to say there’s too much emphasis placed on the degree of adaptation of species to the climates where they live. We assume that this adaptation – if all species are descended from a single form – must have taken place over millions of years, yet a large number of plants and animals brought from different countries remain perfectly healthy in their new home. Also, there are several examples of animal species that have extended their range, within historical times, from warmer to cooler latitudes and the other way round. Rats and mice provide good examples: they have been transported by man to many parts of the world and now have a far wider range than any other rodent, and they can be found living in the cold climate of the Faroe Islands to the north through the tropical zones to the Falklands in the south. It is possible to see adaptation to any climate as a quality that is part of an inborn flexibility of the physical and mental constitution of most animals. Therefore, the ability to survive in the most different climates by both man and his domestic animals, and the fact that elephants once existed in an ice age while living species live in tropical areas, should not be seen as deviations from the rule, but as examples of this flexibility being brought into action under particular circumstances.