34 ESSAYS OF A BIOLOGIST 



in scientists by philosophers, and ridiculed in both by 

 satirists, of hanging or at least disposing of a difficulty 

 by giving it a long name. 



Let us think of the condition of life on earth 

 at any given moment of her evolution. Certain 

 possibilities have been realized by her — others have not. 

 To take a trenchant example, before the Carboniferous 

 or thereabouts, the vertebrates had not realized their 

 possibilities of terrestrial existence — nearly half the 

 globe's surface lay waiting to be colonized by back- 

 boned animals. The earth's surface was conquered 

 then — but the air remained unsubdued before 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 parasites, such and 

 such a hot spring or salt lake is without tenants ; 

 but also improvements can be made in existing types 

 of organization — a tapeworm could be more firmly 

 attached, a salt-lake shrimp could tolerate 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 creatures ; 

 it also involves the conquest of new regions of en- 

 vironment, and so in a real sense the occupation of a 

 new biologic niche. 



In any case, the changes which would confer 

 advantage in the struggle for existence may take place 

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



