54 THE WANDERINGS OF ANIMALS [CH. 



better than specimens taken from a high mountain 

 down into the tropics. We ourselves experience the 

 same effects. No doubt more diseases are rife in the 

 tropics, especially bacterial and parasitic. But this 

 does not go to the root of the problem. A tropical 

 creature, or plant, transported into a cold region, 

 may live, but it will not breed, or ripen its seeds. 

 A cold country species introduced into the tropics is 

 much more likely to die from the. shock, but if it lives 

 it will breed. Many tropical plants can be cultivated 

 in temperate countries where they have to adapt 

 their economy to shorter summers, whilst northern 

 plants, subjected to tropical conditions, are mostly 

 failures since they exhaust themselves through want 

 of rest. Annuals seem naturally to have better 

 chances than perennials. But nature's way of alti- 

 tudinal spreading is slow and steady, not by sudden 

 transports. 



Cold can be counteracted in many ways by animals, 

 as by more food, motion, shelter, a warmer coat, etc. 

 But transference from the changeable temperate to 

 the hot zone with its much more equable climate 

 always implies an excess of warmth over that which 

 the species was accustomed to. 



If the third rule allows of general application it 

 would be of far-reaching importance ; it would allow 

 the further conclusion that a world-wide lowering of 

 temperature has been a most powerful factor in the 



