ADAPTATION 



639 



to "racial senescence." The evolution of 

 the extinct ammonites has been supposed 

 to indicate ultimate extinction through or- 

 thogenesis. The suture lines of the shell 

 chamber partitions with the outer shell un- 

 derwent a remarkable complex develop- 

 ment that probably strengthened the shell. 

 Caenogenetic and recapitulative changes in 

 the sutures have been described. In later 

 evolution the suture line returned to a 

 simpler type, and the shell became un- 

 coiled, both constituting a reversion to 

 more primitive morphological patterns (not 

 necessarily a genetic reversion), an "archa- 

 ism" that is finally associated with extinc- 

 tion. At the end of the Cretaceous, bizarre 

 types with hairpin bends (Hamites) ap- 

 peared, soon to be extinct (Haldane, 

 1932). Huxley (1942, p. 508) warns us 

 that no living ammonites are known, that 

 rate genes might produce some of the 

 described differences in sutures and in coil- 

 ing, that the adaptive significance of these 

 characters or the other effects produced by 

 the modifying genes are unknown, that 

 such a phenomenon as true orthogenesis, 

 predetermined to proceed irrespective of 

 selective disadvantage, is exceptional and, 

 if carried to an extreme, without a known 

 explanatory mechanism. Directional muta- 

 tion at a rate in nature that could markedly 

 overcome counterselection is not known. It 

 would thus be wise to await more data 

 before assuming orthogenetic exceptions to 

 the general adaptive evolution of organisms. 



Racial senescence independent of 

 changes in the phvsical and biotic environ- 

 ments, which would cause extinction of the 

 specialized organism, would seem to be a 

 highly questionable concept (Simpson, 

 1944, pp. 26, 31; also see p. 680, Chap. 

 34). However, harmful characters may 

 evolve even greater harmfulness. The genes 

 favoring rapid growth of such a character- 

 istic as pollen-tube growth might be se- 

 lected over competing genes producing 

 slower growth rates, and at the same time 

 might carrv other characters harmful to 

 the adult that would be augmented by 

 continuous selection of the earlier function 

 (Haldane, 1932, p. 124). 



The future cannot influence present se- 

 lective pressures. Selection mav eliminate 

 individuals with characters that later might 

 save the species from extinction. So-called 

 "racial senescence" or "species senescence" 



may often be the hindsight of the biologist 

 who detects specialization unsuited to the 

 later changes of the environment (Simpson, 

 1944, p. 176; Cain, 1944, p. 241). A spe- 

 cies may be suited to physical conditions, 

 but unable to compete eflFectively with bet- 

 ter adapted species. But this relative de- 

 crease in adaptability is not comparable to 

 physiological aging of an individual organ- 

 ism. In biological evolution, organisms ad- 

 just to repeated events. Order of nature 

 lies at the basis of heredity, develop- 

 ment, and adaptation. Not only do in- 

 dividual organisms "anticipate" future func- 

 tions in their development and organiza- 

 tion, but population systems may likewise 

 be prepared, though they are only pre- 

 pared for conditions that are continuous or 

 repeated. Sexual organisms evolved, not be- 

 cause asexual reproduction was immediately 

 inferior, but because a great degree of var- 

 iation within a conservative system enabled 

 sexual species to compete successfully 

 against those not having the capacity to re- 

 combine genetic elements. 



The problems presented by these seem- 

 ingly teleological phenomena are resolved 

 by an understanding of natural selection 

 acting upon various levels of organic inte- 

 gration (Hutchinson, 1948). If a group of 

 organisms becomes adjusted to relatively 

 stable factors in the environment, it mav 

 evolve slowly, in contrast to the rapid evo- 

 lution of groups adapted to rapidly chang- 

 ing factors, and mav be far less likely to 

 become extinct. Tertiary genera of Pelecy- 

 poda survive at the present time far in ex- 

 cess of Tertiarv genera of Camivora (Simp- 

 son, 1944, p. 27). 



Capacity for plastic behavior, which 

 changes in response to experience, evolved 

 in certain forms because similar initial 

 learning situations were repeated. Without 

 the repetition of events, the capacity for 

 conditioned and learned behavior would 

 not evolve. The evolution of the cerebral 

 cortex in vertebrates illustrates selection of 

 learning capacitv and intelligence. With the 

 growth of scientific prediction, man has ac- 

 quired the greatest capacitv for anticipat- 

 ing the future. He can often make the 

 necessary adiustments in advance and con- 

 trol the conditions. There is little doubt that 

 selection sorts units of social heredity and 

 variation (nongenetic) in a manner some- 

 what similar to its guiding eflFect on ger- 



