ADAPTATION 



631 



monly have breeding seasons initiated by 

 photoperiodicities are able to reverse their 

 reproductive cycles if transported to the 

 south temperate region from the north (p. 

 124). Tropical mammals that have not 

 evolved under the light conditions of tem- 

 perate latitudes, however, maintain their 

 breeding seasons when transported to tem- 

 perate zoological parks (Bedford and Mar- 

 shall, 1942). Such diflFerences in individual 

 adaptability probably result from different 

 gene patterns. The germ plasm allows a 

 plasticity of reaction that enables the in- 

 dividual to respond diflFerentially to a va- 

 riety of conditions. The adaptive capacity 

 of the system rests upon a genetic basis, but 

 the environment may stimulate or limit its 

 expression (pp. 639 and 664). 



The genetic system may also initiate the 

 development of a capacity to control the 

 internal environment in the face of ecologic 

 fluctuations. Edwards and Irving (1943) 

 report that the sand crab, Emerita talpoida, 

 maintains a fairly constant metabolic rate 

 in both winter and summer on the Massa- 

 chusetts shore and continues growth and 

 activity in the winter. This ability to stabi- 

 lize internal conditions within the organism 

 is referred to as homeostasis and is clearly 

 the result of adaptive evolution (Cannon, 

 1941; see also p. 672). 



All organisms exhibit hereditary fitness to 

 the environment and hereditary adjustment 

 of one part of the organism to other parts. 

 We may roughly divide these types of evo- 

 lutionary adaptation into exoadaptation and 

 endoadaptation, but there is no sharp line 

 separating the two categories (Sinnott, 

 1946). Theoretically there is no difference 

 in the basic general causation of either 

 type. Different organismic levels have in- 

 corporated the external environment of the 

 lower levels of individuality into the inter- 

 nal environment of the higher levels (p. 

 683). 



Ecologists are primarily interested in 

 adaptation to the external environment, 

 while physiologists are concerned with the 

 balance and division of labor within the 

 organism which are necessary for the sur- 

 vival of the whole unit. Because of the 

 overlap of these tvpes of adaptation, it is 

 often impossible to make a sharp classifi- 

 cation. Inasmuch as the development of an 

 exoadaptive organ, such as the wing of a 

 bird, involves geologic time, heredity, em- 



bryologic development, physiologic mech- 

 anisms, neuromechanisms, and behavior 

 patterns, the ecologists must lean heavily 

 upon investigations in various subjects for 

 a broad perspective on ecologic adjustment. 

 Likewise, as environmental selective fac- 

 tors are responsible for the slow evolution 

 of such an organ as a bird's wing, other 

 scientists must incorporate ecologic inves- 

 tigations into their search for fundamental 

 principles. Adequate study of a single 

 adaptive organ may rest upon all the main 

 fields of biology and many phases of phys- 

 ical science (Miller, 1937; Howell, 1944). 



A simple example of the overlap between 

 endoadaptation and exoadaptation is af- 

 forded by animals that roll themselves into 

 balls, an action that protects them from 

 predaceous enemies and also may conserve 

 moisture in dry habitats. The best-known 

 example is that of the armadillo, Tolypeutes 

 contirus, whose hard plates on the top of 

 the head and tail close the opening left by 

 the rolled-up body. All parts of the body fit 

 together in the rolled position to make a 

 smooth, hard surface. Other examples of 

 this adaptation are seen among the beetles 

 of the genus Acanthocenis (Acanthoceri- 

 dae), beetles of the genus Agathidium 

 (Silphidae), isopods of the families Arma- 

 dellidiidae, Tylidae, Sphaeromidae, and 

 Cubaridae (Vandel, 1942; Van Name, 

 1936, p. 282), certain mites (Hoploderma- 

 tidae), and certain fossil trilobites. 



These animals have convergently (p. 

 666) evolved structural adaptation between 

 the front part of the body and the rear 

 part, as well as, in special cases, between 

 other parts, including legs or tail. During 

 development, these portions are not me- 

 chanically connected, and yet the morpho- 

 logical outlines grow with exactness toward 

 their ultimate function. Only the adult 

 stage of the beetle, Acanthocenis, can form 

 a ball, while the grub shows no such adap- 

 tation. No function other than protection 

 is evident. Consequently, we find parts of 

 the bodv adapted, even after some delay, 

 to fit other bodv parts to produce a total 

 function that is exoadaptive. 



Several aspects of the organism, such as 

 shape, size, color, and behavior, may be 

 involved in functional adjustment. For ex- 

 ample, Thomas (1941) studied the be- 

 havior of geometrid caterpillars that resem- 

 ble twigs and found stretching and cata- 



