i.8 INCREASING COMPLEXITY OF LIFE 9 



produce further divergences by use and disuse. Limitations of inter- 

 mating may occur on account of limitation of movement, accentuated 

 by partial and, perhaps, eventually complete geographical barriers. 

 Such variations in external circumstance become matched by diver- 

 gences in type, until two new races are produced, at first relatively 

 and then absolutely infertile, so that there are then two separate 

 populations or species instead of one. 



7. The increasing complexity of life 



The acquisition of new matter, and hence growth and reproduction, 

 occurred in the earliest animals by relatively simple means, as it still 

 does today in the bacteria, lower plants, and some protozoa. It is not 

 easy to provide rigid criteria for the definition of 'simple'; perhaps 

 some of the chemical changes involved may be quite complex, but the 

 whole system can, with meaning, be said to be simple. The number 

 of parts that it contains is relatively small and the number of 'adap- 

 tive' actions that it can take is limited. A population of bacteria in a 

 suitable culture medium obtains its raw materials by diffusion; the 

 chief device that it uses to secure these materials is to provide a large 

 number of spores, so that some may come to rest in suitable sur- 

 roundings. Such a life can be said to be more simple than that of a 

 vertebrate, whose system includes many special devices for obtaining 

 access to the raw materials that it needs. We can say that a species of 

 bacteria transmits less information than a vertebrate. LInfortunately 

 there are no satisfactory counts of the number of genes available; the 

 amount of DNA in bacteria is said to be about 0-05 mgm per gm 

 and in rat liver 2 mgm per gm. Bacteria of any one species are able to 

 alter their enzymes to suit the substrates available, but their life does 

 not depend upon the differentiation into numerous cell types each 

 with its special functions. The variety of information available in the 

 'higher' genotypes enables them to take actions that ensure survival 

 under conditions where the 'lower' organisms would die. Of course, 

 each type has its own special 'niche' and the comparison of higher and 

 lower is only possible if we can show exact quantitative differences. 



8. The progression of life from the water to more difficult environ- 

 ments 



In general, the new environments colonized have involved ever 

 wider departures from that watery one in which life first arose. This 

 is shown most strikingly if we contrast the simple way in which the 

 means of life are obtained by a marine bacterium with the complicated 



