Effect of Temperature. 127 



autumn, designated respectively summer and winter eggs, and these two 

 kinds of eggs when hatched turn out animals quite unlike each other. 



The history of the plant louse Aphis is also very remarkable. Tin- 

 eggs hatched in the spring produce only females. These females lay 

 eggs without male impregnation which likewise hatch out only females, 

 and this parthenogenetic process continues till the cool weather of fall. 

 Fourteen generations of unmarried females have been produced thus in 

 one summer. The cool weather of autumn causes some of the eggs to 

 hatch males, and the last batches of eggs are fertilized by these males. 

 These are called winter eggs. They endure the vicissitudes of the win- 

 ter and are hatched in the spring to repeat the process. This partheno- 

 genetic process can be kept up indefinitely. Reaumur produced over 

 fifty generations thus without males in the course of three or four years 

 by supplying artificial warmth in winter, with proper food. 



It is known that the checking of plant growth, which happens to some 

 by the heat of summer and to others by the cold of autumn, is followed 

 by the formation of the seeds and the pushing forward of the buds for 

 the next year. 



The very general mating of animals in the spring in the temperate zones, 

 is, no doubt, very largely due to the effect of the preceding cold of win- 

 ter, in checking the general growth and diverting the vital energies into 

 the channel of reproduction; so that, when upon the rising temperature 

 of spring there is a renewal of activity, it first takes that form. 



This inference is strongly confirmed by the fact that the periodicity 

 of reproduction is quite lost amongst men and some of the domestic 

 animals who are artificially secured against the effects of climatic 

 changes. And it does not obtain in latitudes of even climate the }^ear 

 round. 



In the Phillipine Islands Semper cotfld at all times, summer and 

 winter, find eggs, larvae and propagating individuals of the land mol- 

 lusca, insects and other land animals. In man sexual maturity is at- 

 tained earlier in the tropics. In Egypt and Cuba girls are mature at 

 twelve. Swine also mature early; a boar at Manilla being capable of 

 reproduction at the age of three weeks. It appears to me more proba- 

 ble that this early maturity is not due to the shock or check of sudden 

 heat, such as might and does take place in the temperate zones upon 

 every change from spring to summer. The climate is even and gener- 

 ally prolific, and pushes all sorts of organic bodies to rapid completion, 

 since the}' can grow both winter and summer, and this maturity of the 

 reproductive powers comes here as elsewhere on the heels of the others 

 and after they cease to absorb all the vital energy; but in this case be- 

 cause they arc mature, while in the other cases it is because they are 

 checked while yet immature. 



