FXOLOGICAL 



51 



or eliminative action of the environment. Organisms compete 

 with one another — the most familiar form of the struggle for 

 existence — but as Darwin insisted, the struggle includes the rela- 

 tions between organisms and the inanimate environment. As he 

 said, the plant is struggling at the edge of the desert, but not 

 necessarily with any other plants. The struggle for existence in- 

 cludes all the answers-back that living creatures make to environing 

 difficulties and limitations, and in an automatic way the environ- 

 ment is a sieve as well as a stimulus. Its sifting is often dis- 

 criminate, and this makes for the establishment or improvement of 

 adaptations. 



But we must not think of Nature's sifting too fatalistically, as 



Fig. 17. 



Adaptation in a Barnacle. To the left three common Ship Barnacles (Lepas 

 anatina) attached to a piece of floating timber. From a specimen. The 

 animal proper, at the end of the long stalk (ST), often six inches in length, 

 is encased in its substantial 5-valved shell (SH), and protrudes five pairs 

 of biramose thoracic limbs (C) (the cirri of the Cirripede), by which it 

 wafts microscopic food into its mouth. The other species, the floating 

 barnacle {Lepas fascicularis) , attaches itself to floating seaweed or the 

 like, and obviates the danger of dragging this down, beyond the surface 

 zone of nutritive abundance, by secreting a buoy (B). 



if organisms were always like helpless fishes, around which the 

 environmental net closes, only the little ones escaping through the 

 meshes. On the contrary, in varying degrees living creatures are 

 agents ; they thrust as well as parry ; they act on their surroundings, 

 modifying them; they are ever seeking out new environments and 

 conquering them. 



Thus, without going farther, we see that the range of relations 

 between the living creature and its surroundings is complex, 

 including: 



