Seasonal Changes 



once a day, when the temperature of the water in which 

 it lived was between 7-ioC. ; with an increase of 

 temperature to io-i5C. division occurred twice a day ; 

 at I5-20C. thrice; at 2O-24C. four times, and at 

 24-27C. five times. At the last temperature one 

 individual in four days became the father of a million, 

 in six days of a billion, in seven days of one hundred 

 billions, weighing one hundred kilogrammes. In short, 

 cold lessens vitality ; its usual accompaniment also is a 

 reduced food supply. 



Nowhere in the animal kingdom can the combined 

 influence of warmth and plentiful food, as opposed to cold 

 and a deficient food supply, be studied to more advantage 

 than in the case of the green-flies or aphides. In the 

 summer, when the weather is warm and food abundant, 

 these creatures multiply in a phenomenal manner and at 

 an extroardinary rate. All the individuals are females 

 and bring forth their young without the aid of males the 

 most rapid form of animal increase. Every few hours, for 

 days on end, the mother aphis produces her young. So 

 astonishingly rapid is the increase that, accidents apart 

 that is to say, if all her offspring survived she would 

 be at the end of one year the mother of a family whose 

 combined weight would more than equal that of 

 500,000,000 men. 



With the advent of winter, food becomes scarce and the 

 temperature is reduced ; as a result, males appear and the 

 birth-rate is reduced so rapidly that, were the aphides 

 but human, columns would be written in the newspapers 

 suggesting wild remedies and drawing attention to the 

 serious state of affairs. But the return of summer sets 

 the ball rolling again for the aphides ; in fact, certain 

 specimens which were kept in a greenhouse, with plenty 

 of warmth and abundance of food, never slackened their 

 family affairs for a single day during a period of four 

 years. 



The question of food supply is a very important one 



222 



