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 their leaves during the day and closing them during the night. Again, the brown stoat regularly becomes the white ermine during the winter months.

(b) Where the environment is not regular in its influence, the temporary alterations in the organism to which it leads may be simply adjustments of longer or shorter duration. "The warm-blooded bird or mammal can within limits adjust its heat-production and heat-loss so that the temperature of the body remains the same whether that of the environment rises or falls."

(c) In some cases the environment may make a permanent and indelible impression on the living creature. A change in the environment, be it sudden or gradual, may occasion modifications in the organism which will remain with it as permanent acquisitions. A storm may blow a tree permanently out of shape, and a few years in the tropics may tan a man for life. In such cases the environment has led to the development by the individual of "acquired characters."

(d) Yet the importance of the environment should not be unduly magnified. We should not think of it as an iron fate. In most cases it does not actually cause changes. It only elicits and restrains. All it does is to afford the occasion on which the creature itself changes. Even in instances of "protective mimicry," where the influence of the environment is most immediately apparent, the environment only supplies the stimulus in response to which the organism changes itself. "A green frog, if he is not among green leaves, but amid dull, colourless