74 PROBLEMS OF EVOLUTION 



much owing to the cold as to the difficulty of getting food. In 

 England droughts are not as a rule prolonged enough to bring 

 distress to wild animals, but in other countries they are among 

 the greatest exterminators. An Indian famine leaves a great 

 dearth of animals in the districts that have suffered. I have 

 read of the rush of wild animals in Africa in times of drought to 

 the pools still remaining in the river-courses. Only the more 

 vigorous succeed in reaching the water or are able to hold their 

 own among the herds maddened with thirst whom they find 

 there. In some regions famines and droughts are entirely absent, 

 but none the less there are crises which eliminate the weaker of 

 each species or those less adapted to their surroundings. Of 

 such regions of the earth the equatorial are perhaps the best 

 example. 1 



In three years at Batavia, on the north-west coast of Java, the 

 range of the thermometer was only 27 Fahr., the maximum 

 being 95, and the minimum 68. With us the thermometer will 

 rise to over 80 in summer, and in winter descend till it is not 

 far from zero. This gives a range of some 80 degrees. Then 

 in our islands there are great variations in the amount of food 

 obtainable by wild animals. In the equatorial zone the amount 

 is always large, and does not vary very much in amount. 



In such regions physical conditions being uniformly favour- 

 able evolution has been due almost entirely to the interaction of 

 species. For an individual a crisis comes when he is pursued by 

 an enemy for a species, when some rival species devours all the 

 food that used to be shared ; or when it finds that its old 

 enemies, that have long hunted it, have improved in eyesight, 

 in quickness, in cunning, or in weapons of attack. Here too, 

 therefore, there are crises and pauses, and the struggle for each 

 individual is not incessant. 



The howling monkeys, whose cries were so hideous and 

 depressing to Mr Bates, no doubt have a great fund of vitality 

 that is only wanted for serious business on special occasions. 

 The same is true of the parrots, the toucans (ramphastos), the 



1 For an interesting account of the equatorial regions see Dr A. R. Wallace's 

 Tropical Nature. 



