cH. XIX Influence of Habits and Surroundings 313 



in the course of a year be a progeny which would weigh 

 down 500,000,000 stout men. But all through the 

 summer these Aphides are wholly female, and therefore 

 wholly parthenogenetic ; no males occur. In autumn, how- 

 ever, when hard times set in, when food is scarcer, and the 

 weather colder, males are born, parthenogenesis ceases, 

 ordinary sexual reproduction recurs. Moreover, if the 

 Aphides be kept in the artificial summer of a greenhouse, 

 as has been done for four years, the parthenogenesis con- 

 tinues without break, no males being born to enjoy the 

 comforts of that environment. Periods of fasting occur 

 in the life -history of many animals, and these are very 

 momentous and progressive periods ' in the lives of some, 

 for the tadpole fasts before it becomes a frog, and the 

 chrysalis before it becomes a butterfly. Lack of food, how- 

 ever, may stunt development, as we see every day in the 

 streets of our towns. 



(c) Radiant Energy. — Of the forjns of radiant energy 

 which play upon the organism, we need take account only 

 of heat and light, for of electrical and magnetic influence 

 the few strange facts that we know do not make us much 

 wiser. 



We know that increased warmth hastens motion, the 

 development of embryos, and the advent of sexual maturity. 

 An Infusorian (StylonicMd) studied by Maupas was seen 

 to divide once a day at a temperature of 7°- 10° C, twice 

 at io°-iS°, thrice at l5°-2o°, four times at 2o°-24°, five 

 times at 24°-27° C. At the last temperature one Infusorian 

 became in four days the ancestor of a million, in six days 

 of a billion, in seven days and a half of 100 billions, weigh- 

 ing 100 kilogrammes. By consummately patient experi- 

 ments, DaUinger was able to educate Monads which lived 

 normally at a temperature of 65° Fahr., until they could 

 flourish at 158° Fahr. 



Cold has generally a reverse action, checking activity, 

 producing coma and lifelessness, diminishing the rate of 

 development, tending to produce dwarf or larva-like forms. 

 The cold of winter acting through the nervous system 

 changes the colour of some animals, like Ross's lemming, 



