THE INSURGENCE OF LIFE 159 



and though they flourish most at low or inconsiderable 

 levels in the tropics they extend into the warmer temperate 

 regions, and are also quite common at considerable eleva- 

 tions in the torrid zone.' As a particular example, he 

 takes Paratelfhusa spinigera, which is very common in the 

 swamps of Lower Bengal. 



' In the rainy season it can be seen in any Calcutta tank, 

 often reposing on the bank, half immersed in the water : 

 in the cold season it may be found in the jheels in swarms, 

 haK-buried in the mud : in the hot season, where the sur- 

 face waters dry up, it digs deep burrows to get down to the 

 ground water. The same species, P. spinigera, on the one 

 hand, ascends the Ganges and Jumna as far as Hardwar 

 and Saharanpur, and the Jhelum valley to an elevation of 

 two thousand feet, and, on the other hand, does not object 

 to the brackish water of the Gangetic delta.' 



The Biology op the Seasons 

 We have given some examples of what might be called 

 the conquest of space — the exploitation of the earth, the 

 making the best of difficult conditions, the circumventing 

 of obstacles ; but there are other instances of the same 

 quality of hie which might be grouped under the title 

 the conquest of time — the victory over temporal vicissitudes. 

 This is in great part the theme of a previous study ^ and 

 we shall confine ourselves here to a few illustrations. 



The general problem is how living creatures suit them- 

 selves to the external periodicities of the seasons, or of 

 day and night, or of oscillations of chmate. In diverse 

 ways the internal rhythms of life have come to be adjusted 

 to the external periodicities. It is said that the tropical 

 1 The Biology of the Seasons, 1911. 



