NATURAL SELECTION 



671 



cised; discrimination is shown by the ejec- 

 tion or destruction of the egg or by deser- 

 tion of the nest; the evolution of resem- 

 blance between the eggs of cuckoos and 

 those of their normal foster parents is 

 brought about by the constant destruction, 

 by the latter, of the cuckoos' eggs most un- 

 like their own, and the survival of those 

 most like them (also see p. 615), 



Adaptive radiation of a single phyloge- 

 netic Une into various habitats and the con- 

 vergent evolution of many lines within the 

 same habitat amply illustrate the slow 

 effect of environmental selection pressures. 

 Both the genetic complexity of adaptation 

 and the subtlety of the ecological factors are 

 evident. Mimetic resemblance has been a 

 controversial problem in biology for a long 

 time. This concept is still in need of more 

 thorough analysis. Nevertheless, mimicry 

 exemplifies the convergent evolution of spe- 

 cies through natural selection with a con- 

 vincing or at least a supporting quantity 

 of factual material. 



ECOLOGICAL POSITION 

 AND HOMEOSTASIS 



Once an organism has evolved to fit a 

 particular combination of ecological factors, 

 the maintenance of ecological position be- 

 comes an important aspect of survival and 

 adaptation. As a result of environmental 

 changes and periodicities such as seasonal 

 fluctuations, certain ecological factors may 

 move to another region, thus tending to 

 leave the organism in an unfavorable habi- 

 tat. Orientation behavior (p. 648) and lo- 

 comotion are commonly adaptations that 

 keep the animal within a moving favorable 

 environment. Both the abiUty to stay within 

 a favorable stable habitat and the ability to 

 move with the shift of favorable conditions 

 are adaptations for maintaining ecological 

 position. 



Many organisms hold ecological position 

 by means of various adaptations for attach- 

 ment. Examples are the suckers, thread, 

 and streamlined shape of the larva of the 

 black fly (Simuliidae) living in rapid 

 water where the danger of being swept into 

 the unfavorable pond environment is ever 

 present. The danger of dislodgement is 

 overcome by a great variety of animals ad- 

 justed to life on rocky sea shores by means 

 of attachment organs or boring devices. In 

 this connection, it is noteworthy that the 



holdfast cell is the first speciahzed somatic 

 cell of the simple filamentous algae. The 

 attachment organ of the large brown algae 

 (kelps) is particularly well developed. 



If the adaptations for maintaining ecolog- 

 ical position fail, great mortality in the un- 

 favorable environment is to be expected. 

 This expectation is borne out by studies of 

 elimination under extreme conditions at the 

 borders of habitats and among accidentally 

 dispersed individuals (Storey, 1937; Mil- 

 ler, 1940; Dendy, 1944; see also p. 634). 



Stable environments without great fluc- 

 tuation in conditions are naturally favorable 

 habitats for organisms, provided the essen- 

 tials for life are available. The deep sea and 

 the cave environment are particularly stable 

 in their physical conditions. Organisms have 

 moved into these habitats in their phylo- 

 genetic history in spite of the absence of a 

 primary plant food supply. 



Habitats affording proper food and ef- 

 fective protection from predatory and 

 parasitic enemies may be favorable for a 

 given species, whereas other areas with 

 similarity of physical conditions may be un- 

 tenable. The pika (Ochotona princeps) of 

 western North America lives only in talus 

 slopes which combine protection from pred- 

 atory enemies with dry shelter for the 

 storage of their plant food. These rodents 

 do not move more than thirty feet away 

 from the talus environment (Hall, 1946, p. 

 49). 



Certain desert reptiles are limited in 

 their toleration to a range of temperature 

 that is exceeded in their general habitat. 

 These reptiles often show physiological ad- 

 justment to extreme temperatures, but they 

 move to shade or burrow when the ground 

 surface temperature approaches the toler- 

 able limit (Cowles and Bogert, 1944). 



Migrating birds find a favorable climatic 

 environment by moving north and south 

 with the change of seasons, or up and 

 down in mountainous districts. It is inter- 

 esting to note that the stimulus initiating 

 migration of some birds is the length of 

 daylight, probably acting through the pitui- 

 tary gland secretory effect on the gonads 

 and thence to behavior responses. Survival, 

 however, is not determined by the direct 

 response to light, but rather by climatic and 

 food factors. 



In the relatively stable temperature con- 

 ditions of the tropics migration may be cor- 



