34 ESSAYS OF A BIOLOGIST 



bilities have been realized by her — others have not. 

 To take a trenchant example, before the Carbonifer- 

 ous or thereabouts, the vertebrates had not realized 

 their possibilities of terrestrial existence — nearly half 

 the globe's surface lay waiting to be colonized by 

 backboned animals. The earth's surface was con- 

 quered then — but the air remained unsubdued be- 

 fore the mid-Secondary. In every period, there must 

 be not only actual gaps unfilled in the economy of 

 nature — such and such an animal is without para- 

 sites, such and such a hot spring or salt lake is with- 

 out tenants; but also improvements can be made in 

 existing types of organization — a tapeworm could be 

 more firmly attached, a salt-lake shrimp could toler- 

 ate an even higher concentration of brine. 



These two sorts of possibilities really overlap. For 

 instance, an increased efficiency of vision must be 

 an improvement in pre-existing structures and crea- 

 tures; it also involves the conquest of new regions 

 of environment, and so in a real sense the occupation 

 of a new biological niche. 



In any case, the changes which would confer ad- 

 vantage in the struggle for existence may take place 

 in any direction — with, or against, or at right angles 

 to the stream of progress. By means of those which 

 march with that stream, the upper level of life's 

 attainment is raised. But the struggle still goes on: 

 and again, starting from this new condition, there 

 will be variations in every direction which will have 

 survival value, and some of these will be progressive; 



