io EVOLUTION OF LIFE in- 



activities that go to maintain a man alive in a city. Yet all living 

 systems, even those that have changed most markedly since their first 

 origin, are still watery, and must have salt and nitrogenous com- 

 pounds with which to make proteins and so on. Perhaps, indeed, the 

 basic plan of the living activity differs less in the various types than 

 one might suppose. 'Protoplasm' is certainly not identical in all 

 creatures, but it may be that it differs less than do the outward forms 

 that support it. 



In order to provide the conditions necessary for the maintenance of 

 such a watery system, in very different environments, many auxiliary 

 activities have been developed. It is these that give added complexity 

 to the higher animals and plants, enabling them to undertake what 

 can be called more difficult ways of life. In order to do this their 

 activity must also be physically greater than is necessary in more 

 lowly types. It may be presumed that more energy is transferred to 

 maintain a given mass of living matter in the less 'easy' environments, 

 and in this sense the higher animals are less efficient than the lower, 

 by a very crude criterion of efficiency. 



According to this conception, then, evolution has involved a change 

 in the relationship between organism and environment. Life has 

 come to occupy places in which it did not exist before. Perhaps the 

 total mass of living matter has thus been greatly increased. It must 

 not, of course, be supposed that every evolutionary change has pro- 

 duced an increase in complexity in this way; examples of 'degenera- 

 tion' are too well known to need quoting. We have, however, a clear 

 impression that through the years there has been, in general, some 

 change in animals and plants and that in a sense some of the later 

 organisms are 'higher' than the earlier. It is hardly possible to deny 

 that there is some meaning in the assertion that man is a higher 

 animal than amoeba. Our thesis attempts to specify more clearly what 

 we can know about this evolutionary change, by saying that it con- 

 sists of a colonization by life of environments more and more different 

 from that in which life arose. This colonization was made possible 

 by the gradual acquisition of a store of instructions enabling adjust- 

 ments to be made by which life could be maintained in conditions not 

 tolerable before. 



It is not easy to enumerate the complexity of any animal or to 

 define quantitatively the nature of its relations with its environment, 

 and for this reason it is difficult to prove our thesis rigorously. This 

 book nevertheless makes an attempt to show how the organization of 

 vertebrate life has become more complex since it first appeared, and 



