22 EVOLUTION OF LIFE i. 10 



have produced all the varied types. The pattern of organization we 

 have to study is that of the animal as an active system maintaining 

 itself in its environment. This tendency to maintenance and growth 

 is the central 'force' that produces the variety of life. The opportunity 

 for change is provided by the fact that reproduction seldom produces 

 an exact copy of the parent, and thus a range of types is provided. 

 The tendencies to grow and to vary lead animals to colonize new en- 

 vironments and produce the variety of life. As evolution has proceeded 

 animals have come to occupy environments differing ever more widely 

 from the sea in which life probably arose. Life in these more difficult 

 environments is made possible by the development of special devices, 

 making the later animals more complex than the earlier and in this 

 sense 'higher'. It remains uncertain what influences have been respon- 

 sible for producing the changes in organic form. Geological evidence 

 shows that there have been many changes in climate and geography, 

 some of them proceeding at very slow rates in comparison with the 

 rhythms of individual animal lives. It is uncertain whether evolution- 

 ary changes follow these slow geological changes, or are a result of 

 the instability imposed on living things by climatic rhythms with 

 shorter periods, such as those of days, years, and the sunspot cycles. 



