You do not currently have permission to view the full text for this document. Please sign in to an account with an active subscription or contact support.
The living organism can tolerate relatively little change in its environmental circumstances—not only because altered circumstances have a noxious effect—but because the organism has learned to utilize its environment for its own needs.
It is not sufficient to say that the terrestrial organism tolerates oxygen or that it requires oxygen for survival. The terrestrial organism has developed an oxygen conveying and enzymatic respiratory mechanism which utilizes oxygen provided at the partial pressure which prevails on the surface of the earth. A substantial change in that partial pressure in either direction has deleterious effects. Similarly the photosynthetic process permits green plants not only to tolerate but to utilize atmosphperic carbon dioxide.
Physical aspects of the environment too, are employed to the advantage of the organism. Aquatic animals not only tolerate water but depend on it for locomotion and circulation and conveyance of metabolites. Birds not only manage heights but become adept at learning to drop hard objects (e.g. clams) in order to break them or to frighten intruders (Tinbergen). Locally prevalent colors are used to camouflage. Nest construction utilizes available building materials with seeming ingenuity. An especially striking instance of turning a necessity to advantage is parasitism.
It is not only external necessity which the organism turns to its own purposes; it exploits the data of its own constitution. Phylogenetic studies have abundantly demonstrated that evolutionary advance is accomplished by the direction of old embryonic anlage to new forms and functions. The differentiation of gill slit primordia to a series of head and neck structures is one well known example. The assignment