236 



ANALYSIS OF THE ENVIRONMENT 



Much the larger segment of biotic pressure 

 is to be seen in the influence of predator 

 animals. The relation of plant evolution to 

 the animals that feed upon plants is evi- 

 dent in the innumerable animal-repellent 

 devices, in the physiological and popula- 

 tional adaptation of the development of a 

 surplus, and finally in animal-attracting de- 

 vices when a surplus food material exists 

 and some benefit is derived by the plant 

 from the animal members of the association. 



The relation of a food animal to its pred- 

 ator environment is equally evident. It may 

 be referred to under the concept of preda- 

 tion pressure, and this relation results even 

 more clearly in varied evolutionary 

 transformations. Broad effects widespread 

 through the animal kingdom that are sum- 

 marized as responses to predation pressure 

 are, on the passive side: protective resem- 

 blance, poisonous or otherwise repellant 

 secretions or quaUties (these often asso- 

 ciated with conspicuous coloration), armor 

 and defensive spines, and high reproductive 

 potential, i.e., a safety factor in population 

 numbers. Active forms may be adapted in 

 the direction of fleetness, of defensive 

 weapons actively used, of intelligence, or 

 again in the direction of liigher reproduc- 

 tive potential. The relation of a host to its 

 parasites falls mainly and necessarily into 

 the passive series, and the only effective 

 responses fie in the development of im- 

 munity to toxins produced by the parasite, 

 in the production of countertoxins or in the 

 more or less incidental growth of sufficient 

 surplus of food or food tissue for the para- 

 site. The attempted active avoidance of 

 parasites by host animals, familiar to farm- 

 ers in the reaction of horses and cattle to 

 their respective botflies, though apparently 

 quite ineffective, shows how such adverse 

 environmental factors may impress the germ 

 plasm with inherited behavior reactions 

 through natural selection. 



Only a few plants other than bacteria 

 and certain fungi such as the Laboulbena- 

 ceae effectively prey upon living animals 

 (see p. 259). Among those that do, many 

 exhibit elaborate structures in the foi-m of 

 traps or pitfalls, with a wide range of com- 

 plexity from the simple sticky pads of the 

 sundew to the spring mechanism of Venus' 

 flytrap, and the simple pitcher of Sarracenia 

 to the elaborate pitcher-leaf (Nepenthes). 

 The abundant aquatic bladderworts (Utri- 

 cularia) tap the supply of minute crusta- 



ceans and insect larvae of fresh waters by 

 means of their submerged traps. The tropi- 

 cal fungi of the genus Cordyceps parasitize 

 and kill caterpillars and even adult insects 

 (Kingston, 1932). The concept of biotic 

 pressure appears again in subsequent chap- 

 ters, in connection with population ecology, 

 community metabolism, and evolution (pp. 

 349 and 648). 



Impact of Food Surplus 



The principle that animal populations 

 tend to be lirnited by their food supply in- 

 volves the corollary that populations tend 

 to expand in the presence of available food. 

 When the Darwinian principle of natural 

 selection is taken into account as a trans- 

 forming influence, it is evident that the 

 development of new species and of new 

 types is to no small extent an evolution to 

 take advantage of unexploited food sur- 

 pluses. Such an evolutionary expansion is 

 notable in animals adapted to severe cfi- 

 mates, hke the Antarctic penguin or the 

 Arctic polar bear, in those adapted to pecu- 

 har conditions Hke those in caves or in the 

 deep sea, and in the return of land animals 

 of various types to fresh-water or marine 

 Hfe. It is our thesis that evolutionary exploi- 

 tation of food surpluses is a far-reaching 

 principle, throwing light on many ecologi- 

 cal problems, and especially pointing to the 

 significance of surplus food in contem- 

 porary adjustments of animals to their 

 environment. 



It appears to be a fundamental attribute 

 of living organisms to tend to use all avail- 

 able food supplies. The vast invasions of 

 new habitats, Uke the conquest of the land 

 by plants in Devonian time, the expansions 

 of land animals in the late Paleozoic, or the 

 reconquest of vast northern areas after the 

 retreat of the glaciers of the Pleistocene, 

 afford illustrations on a grand scale of the 

 response of Hving matter to unused food 

 supplies. Further illustrations may be seen 

 in minor expansions into the smaller habitat 

 niches which often exhibit rigidly adapted 

 organisms (e.g., the commensals of ants and 

 termites) and adaptation to specific levels 

 in food chains and food pyramids (e.g., 

 scavengers monophagous types). 



The evolution of plants involves a great 

 variety of adjustments for the utilization of 

 inorganic food supplies wherever these 

 come in contact with oxygen, carbon diox- 

 ide, and water, with, of course, secondary 



