CHAPTER I 



INTRODUCTION TO CYCLES 



ONE of the most arresting and widely known of 

 biological phenomena is the cyclic breeding habit 

 shown by so many animals and plants. Indeed, it is 

 evident that the lives of such organisms are compounded 

 of greater and lesser cycles of growth and reproduction. 

 In the first place there is the major cycle of the life span 

 itself, which passes from fertilization to embryonic 

 growth, through immaturity to maturity, and so to 

 senescence. Sometimes this is the only cycle in evidence, 

 as when an individual breeds only once and quickly 

 dies. This is rare among the vertebrates, but a well- 

 known example is the fresh-water eel which ends its 

 life when it migrates across the Atlantic to spawn in the 

 Sargasso Sea. 



In most vertebrates the great cycle of life contains 

 within it a series of lesser cycles, the most obvious of 

 which is the- seasonal reproductive cycle. In temperate 

 regions this usually has a duration of one year, and it 

 is common for the major breeding season to be in spring 

 and early summer. However, many species breed at 

 other times of the year, the sheep, for instance, doing so 

 in autumn and the cod in winter. Sometimes this yearly 

 breeding habit is only vaguely defined as when the 

 common dogfish, which spawns most actively in spring, 

 nevertheless lays some eggs in every month of the year.i*^ 

 Commonly, however, the timing of a breeding season is 

 precise so that, for instance, unless the weather be very 

 unseasonal, one can say that the British starling will lay 

 its first eggs during the last two weeks of April. ^* 



For still greater precision in timing it is necessary to 

 turn to the invertebrates where the classical example is 



