Periodicity. 123 



second season. Others last a term of years, producing leaves and seeds 

 each season, which drop off leaving only the stems and roots or perhaps 

 only the roots to retain the vital principle of the individual through the 

 term of years. We can easily see how the limit of annual herbs is com- 

 passed, and how in fact cessation of growth of the leaves and seeds 

 must take place, and how they must fall from the plant in the end of 

 the summer. 



Animals, too, are required to become newly adapted to the change in 

 temperature every fall and spring. Many animals hibernate, or go into 

 a winter sleep, becoming more or less torpid in the winter, in which con- 

 dition their tissues undergo comparatively slow waste, their respiration 

 being much less rapid than in summer. Frogs in winter suspend a large 

 share of their vital activity. Their blood has been proved by experi- 

 ment to be destitute in winter of the diastasic ferment or digestive juice 

 which it has in summer. Its waste and repair are both reduced to a 

 minimum during the cold season. 



In some tropical countries alligators burrow in the mud during the 

 dry season, remaining some months in a state of torpidity or summer 

 sleep, and emerge at the approach of the rainy season. The annual mi- 

 grations of birds, fishes and other animals is also due to the annual 

 periodicity of climatic changes. The necessity of the germination of 

 seeds in the spring is obviously because they need the coming summer 

 for growth and maturation, and this necessity selects such as can do 

 this. The same necessity selects the spring and early summer as the 

 best season for the mating and sexual pairing of many of the animals. 

 Birds by laying their eggs in spring and rearing their young during the 

 summer bring them to a self-supporting condition by winter, so that they 

 can provide for their own wants or are able to migrate to a milder cli- 

 mate with their ciders. The duration of gestation with many of the 

 larger mammals is such from 7 to 11 months that by the pairing in 

 early summer the birth of the young is brought about in the following 

 spring, and the young' animal has its youthful nursing days in the time 

 of year most favorable for its strong and vigorous nourishment. Such 

 an advantage as this would be cause for selection. 



Menstruation is another periodical phenomenon monthly in man as 

 the term implies, and connected with the development of ova. The 

 females of the Cynomorpha (Doucs and Baboons) are subject to the 

 menstrual hemorrhage. According to Buffon this is common among the 

 Ape tribes. Both Apes and men have, however, outgrown the domina- 

 tion of the seasons in respect to the birth of the offspring, as in all the 

 tribes they arc born at all times of the year. 



But with birds the season appears to he omnipotent, even t<> the reg- 

 ulating of their physiological condition sexually wilh great precision in 



