252 ORIGIN AND NATURE OF LIFE 



their position at day and night, when the 

 plants have been kept in darkness. 



The whole of the physiological and chemical 

 processes of reproduction in the mammalia 

 illustrate the phasic activity of living matter 

 in a most striking fashion. In the lower 

 mammalia, there are the seasonal periods of 

 activity in ordered sequence, the whole 

 reproductive system passes into excitement 

 in its different parts, showing the pre-cestral 

 period, then the active mental and other 

 excitement of the oestrus, followed in turn 

 by the post-cestral period. In the human 

 species, the cycle is even more regular, and is 

 more frequently recurrent. 



During gestation the same phasic intervals 

 are seen, as also in post-natal growth, and 

 throughout the whole life of the individual 

 the same phasic repetition is seen constituting 

 the great cycle of life. 



In the invertebrata cyclic processes are 

 often shown in a most remarkable way. 

 Mention need only be made of the metamor- 

 phoses of the insecta, and the wonderful 

 developmental history of the echinodermata 

 and mollusca. In protozoa also there are the 

 alternations of sexual with asexual (or parthe- 

 nogenetic) reproduction occurring spontaneous- 

 ly, or in response to environmental changes. 



