ADAPTATION 



635 



region of Sanibel Island, Florida (see p. 

 334). These low temperatures destroyed 

 large numbers of tropical species of fishes, 

 while the mortality of the temperate types 

 was not great (Storey and Gudger, 1936; 

 Storey, 1937). Obviously, the fishes well 

 adapted to the normal warm climate may 

 be unable to withstand occasional cold, and 

 the range of the species may thus be 

 limited by the unusual occurrence of ex- 

 treme conditions (see p. 653). Liebig's 

 "Law of the Minimum" (p. 198) as re- 

 stated by Taylor (1934) is as follows: "The 

 growth and functioning of an organism is 

 dependent on the amount of the essential 

 environmental factor presented to it in 

 minimal quantities during the most critical 

 season of the year, or during the most 

 critical year or years of a climatic cycle." 

 Similar relations hold for maximal quantities. 

 Haldane (1932, p. 118) pointed out that 

 ecologists may be able to determine the 

 normal incidence of selection for a given 

 species, but that a great disaster or an emi- 

 gration may make the characteristics of a 

 single survivor important (see p. 604). 



Ancient adaptations may be retained 

 through long periods of subsequent evolu- 

 tion and diversification, and consequently 

 may be homologous within a large group 

 of organisms. A pattern of many different 

 characters, with presumably somewhat dif- 

 ferent genetic bases in the same organism, 

 should indicate homology with the same 

 set of characters found in another organ- 

 ism. Adaptive characters not associated 

 with the pattern of homologies in the pos- 

 tulated common ancestor may be presumed 

 to be analogous. 



If we keep in mind that homology must 

 be correlated with genetic similarities and 

 a common phylogeny, while analogy must 

 be correlated with functional (often envi- 

 ronmental) similarities arising through con- 

 vergence and natural selection, we can 

 avoid many types of error common in the 

 past (Hubbs, 1944). 



Adaptation is a fundamental attribute of 

 all living organisms and is a highly com- 

 plex phenomenon. The explanation of the 

 origin of adaptation is one of the prime 

 problems of biology. Ecologv offers much 

 evidence for the analysis of evolutionary 

 exoadaptation. Special aspects of adapta- 

 tion will be discussed in the following parts 

 of this chapter. The operational mecha- 



nisms guiding organisms toward increased 

 adaptation will be dealt with in the chap- 

 ter on Natural Selection, and the evolu- 

 tionary aspects of complex community ad- 

 justments and of ecosystems will be the 

 subject of the final chapter. 



CAENOGENESIS AND PALINGENESIS 



During ontogeny the environment is of- 

 ten different for the successive develop- 

 mental stages. Embryos and young must 

 survive in environments that may be dis- 

 similar to the surroundings of the adult. 

 This necessitates adjustments that contrast 

 strikingly with those of the adult organism. 

 If the special adaptations of the young are 

 not found in the adult and have evolved 

 more recently than the adult adjustments, 

 they are termed caenogenetic. 



The best examples of caenogenesis are 

 found among the insects that exhibit com- 

 plete metamorphosis. For instance, the 

 larva of a mosquito living in a pond has 

 specialized spiracular openings near the 

 end of the abdomen for gaseous exchange 

 at the surface, an adaptation not needed or 

 found in the adult. The adult, living a ter- 

 restrial and aerial life, uses wings for loco- 

 motion that develop, but are not functional, 

 in the larva and pupa. Feeding adaptations 

 likewise differ markedly in the larva and 

 adult of the same individual with the same 

 genetic constitution. The adaptations of the 

 larva to aquatic life are chronologically 

 more recent in evolutionary history than 

 the wings functioning for flight in the adult. 



A somewhat unusual case of caenogenesis 

 is found in the evolution of the castes of 

 the social termites, which develop by grad- 

 ual metamorphosis. There is little doubt 

 that both soldiers and workers are caeno- 

 genetic, exhibiting many characteristics of 

 nymphs (Fig. 146). The soldier is the only 

 sterile caste in the primitive termites (Kalo- 

 termitidae, Mastotermitidae) and thus orig- 

 inated before the worker. The soldier re- 

 sembles the winged imago more closely 

 than does the worker of the higher termites 

 (Termitidae). The worker seems to be a 

 caenogenetic evolution of soldier nymph 

 characters with specialized behavior not 

 found in either the young or adults of the 

 other castes (Emerson, 1926; see p. 633). 

 Nymphal, larval, or embrvonic charac- 

 ters may be retained in the adult, or the 

 voung stage may be sexually mature, a phe- 



