4 THE BIOLOGY OF THE SEASONS 



Similarly we find in modern studies of plant and animal 

 life in our own country of the shore-fauna or of the flora of 

 lakes, of birds and their nests, or of flowers and their visitors 

 that the biological note is becoming more and more 

 dominant. In some cases it is obvious that the evolution 

 idea forms the unifying centre of the study. In the same 

 way, it is by an emphasis on underlying principles that a 

 Biology of the Seasons should differ from a Natural History 

 of the Year. And that is our aim in this book. 



EXTERNAL PERIODICITIES 



The physical fact of most importance in connection with 

 the biology of the seasons is the familiar one that the sun 

 is our main source of energy, and that, according to our 

 seasonal relation to it, we get varying amounts of heat and 

 light. With this primary relation are associated many 

 secondary seasonal variations, in rain-fall, barometric 

 pressure, winds, tides, and so on. The Biology of the Seasons 

 has for its central task an inquiry into the adaptations of 

 living creatures to the external periodicities, which we sum 

 up in a rough, popular way in speaking of Spring, Summer, 

 Autumn, and Winter. The astronomical division of the 

 year is familiar : From the Vernal Equinox to the Summer 

 Solstice ; from the Summer Solstice to the Autumnal 

 Equinox ; from the Autumnal Equinox to the Winter 

 Solstice ; from the Winter Solstice to the Vernal Equinox. 

 But this precise division of the year is not very useful bio- 

 logically, and some compromise must be accepted. 



Thus, for practical purposes, we may agree that in 

 North Temperate regions, Spring is late March, April, and 

 May ; Summer is June, July, and August ; Autumn is 

 September and October ; Winter is November, December, 

 January, February, and part of March. The basal fact, 



