672 



ECOLOGY AND EVOLUTION 



related with fluctuations of food. The pen- 

 nant-winged nightjar (Cosmetornis vexil- 

 larius) of tropical Africa crosses the equa- 

 tor twice each year. It breeds in southern 

 Africa from September to November. In 

 February it migrates northward, often 

 crossing the central African rain forest, to 

 the grass country of Uganda, Uele, the 

 Sudan, and Nigeria. This migration follows 

 the beginning of the wet season, when 

 winged termites, the usual food of this bird, 

 are flying (Chapin, 1916). 



Many organisms have evolved an abiUty 

 to modify their environment in the direction 

 of relative stabiUty. The important phys- 

 iological principle of homeostasis (main- 

 tenance of constant conditions; p. 631) is 

 also an ecological principle. Examples are 

 afiForded by the beavers, which build dams, 

 and by those social insects that build nests. 

 The termite nest functions primarily to 

 maintain a constant high humidity (Emer- 

 son, 1938; p. 428), A careful study of the 

 humidity within a mound nest of an Austral- 

 ian termite (Nasutitermes exitiosus) was 

 made by Fyfe and Gay (1938). They con- 

 clude : 



"In brief, the structures and composition of 

 the mound strongly tend to retain the moisture 

 produced by the metabolism of the termites 

 in the mound, but the temperature maintained 

 by the living termites and the special properties 

 of the mound material prevent tlie deposition 

 of free water in the central regions. The system 

 balances the amount of water produced by 

 metabolism by the amount lost by diffusion 

 and evaporation, and provides a buffer mech- 

 anism to compensate variations in the rate of 

 production and loss." 



Man, by intelligent behavior and scien- 

 tific knowledge, is rapidly increasing the 

 modification and control of his environment. 

 He not only changes the physiographic 

 features of the earth, but profoundly modi- 

 fies the fauna and flora, and in addition 

 develops domestic plants and animals 

 adapted to his needs. He also tends more 

 and more to control his social interactions 

 and to evolve social division of labor and 

 integration. Environmental modifications 

 that are made without ecological knowledge 

 or long-term social perspective may neces- 

 sitate conservation measures if permanent 

 harm or decreased homeostasis is to be 

 avoided. 



Spencer postulated an increasing inde- 

 pendence of the environment accompany- 

 ing increasing complexity of organization. 

 One of the major aspects of adaptation is 

 the control of the external environment 

 through the incorporation of the environ- 

 ment of one organismic level within the 

 system of a higher level (p. 694). Thus the 

 immediate surroundings of each living unit 

 are brought under control and allow an or- 

 ganic evolution of the environment (pp. 

 695, 698). Homeostasis is not only char- 

 acteristic of the cell environment in the 

 multicellular organism and the individual 

 environment in the aggregated species 

 population (p. 694), but to a certain ex- 

 tent it is also characteristic of the evolving 

 ecological community (p. 728). 



REGRESSIVE EVOLUTION 



It has been observed that an evolu- 

 tionary regression of structural adaptations 

 may occur, leaving vestigial organs that 

 often persist long after the function has dis- 

 appeared (see PaHngenesis, p. 635). Ani- 

 mals are known with vestigial eyes, legs, 

 wings, lungs, teeth, mouths, and even 

 heads, as well as innumerable other organs 

 and organ systems. 



Regressive evolution is by no means con- 

 fined to a few forms living in special habi- 

 tats, but is a universal phenomenon. Every 

 living organism seems to have lost func- 

 tional adaptations characteristic of its an- 

 cestors. The environment is involved in this 

 regressive evolution, first, because special 

 ecological selection pressures have de- 

 creased or vanished in certain instances, 

 and, second, because there is a convergent 

 degeneration of analogous as well as of 

 homologous functional structures in differ- 

 ent organisms in similar habitats. 



Sometimes one function of an organ re- 

 gresses and may be replaced by another 

 function. The ears of mammals are com- 

 plex organs with the elements of the middle 

 ear traceable back through the jaw appara- 

 tus of reptiles, amphibians, and fishes to 

 primitive gill-bar structures of the jawless 

 fishes (Westoll, 1943a). In this instance 

 we see a modification of breathing struc- 

 tures into structures for eating and finally 

 into accessory organs of hearing, with a re- 

 duction of some parts and a development of 



