NATURAL SELECTION 



643 



chance mutations and recombinations and 

 that these variations arise initially with no 

 influence by a directive ecologic factor, 

 gives a valid basis for the principle of pre- 

 adaptation. As will be discussed later (p. 

 647), this aspect of preadaptation applies 

 only to changes of simple genetic factors 

 and cannot be used to explain genetically 

 complex and highly adapted characteristics 

 (for a contrary opinion, see Goldschmidt, 

 1945). 



Another aspect of preadaptation is found 

 in the fitness of organisms adjusted to new 

 habitats that have many factors in common 

 with the habitats originally occupied by 

 the ancestral forms. In some cases, new 

 habitats are invaded without evolutionary 

 modification. Many animals and plants in- 

 troduced successfully into a region new to 

 them exhibit such preadaptation. 



Twenty-four specimens of the European 

 rabbit were introduced into Victoria, Aus- 

 tralia, in 1859 and in three years became 

 a pest. They spread rapidly to Queensland 

 and South Australia. This success was pos- 

 sibly because of the similarity of climate 

 and general food in the European and Aus- 

 tralian regions, together with the lack of 

 competition from other placental mam- 

 mals. 



Bats are adjusted to nocturnal hfe and 

 are able to avoid collision during flight in 

 the dark by emitting supersonic sounds 

 (30,000 to 70,000 cycles per second), 

 which rebound from objects in their path. 

 Orientation by this method is called echolo- 

 cation (Griffin, 1944, 1946; Vesey-FitzGer- 

 ald, 1947). Although such adjustments en- 

 able bats to become important members of 

 cave communities, there is no indication 

 that the cave environment as such created 

 any selection pressure that influenced the 

 nocturnal adaptation of bats. 



Ewing (1933) recorded the case of the 

 kangaroo mallophagan {Heterodoxus longi- 

 tarsiis) that belongs to a subfamily (Boo- 

 pinae) all members of which are found on 

 Australian marsupials. Through the agency 

 of man, this biting louse was brought with 

 its host to zoological parks and circuses, 

 where it transferred to dogs and is now 

 frequently more abundant on dogs than 

 the original biting louse of the dog (Tri- 

 chodectes canis). This introduced louse has 

 also been recorded on coyotes (Jellison, 

 1942). 



Other cases of adaptive dispersal into 

 new habitats involve some genetic modifica- 

 tion in addition to older adaptations to 

 environmental factors in both the older and 

 more recent habitat (Gregor, 1944). 

 Hubbs (1938, p. 271) states: "Practically 

 all cave-fishes, and also the blind fishes of 

 other habitats, seem to have had ancestors 

 which to a varying degree were preadapted 

 to successful life in utter darkness." (Ex- 

 ceptions are the Characinidae of Mexican 

 caves according to Breder, 1943; see also 

 p. 674; Fig. 247.) Hubbs also declares (p. 

 272) : "Although the permanent occupa- 

 tion of hghtless caves seems to have been 

 generally made possible by a preadapta- 

 tion to such hfe, there is no good reason 

 to believe that this preadaptation was com- 

 plete. Blind fishes are not known to occur 

 outside of caves, except in other more or 

 less completely dark situations to which 

 the blind forms seem rigidly adapted and 

 inescapably bound." 



It should also be pointed out that loss of 

 a character, such as the eye of cave fishes, 

 is doubtless a far simpler genetic process 

 than the acquisition of eyes adapted to vi- 

 sion (p. 647), so that positive adaptation 

 probably involves a longer period of time 

 than regressive evolution of the same or- 

 gan. Nevertheless, the slow acquisition of 

 adaptive characteristics in one habitat may 

 in part preadapt the organism for another 

 habitat that shares similar ecologic factors. 



As pointed out by Ferris (1943), only 

 those changes can occur that have a pre- 

 existing base. A change can arise and main- 

 tain itself only when and if the stage has 

 been set for it. New adjustments presup- 

 pose previous harmonious adjustments both 

 within the organism (White, 1945, p. 304) 

 and between the organism and its environ- 

 ment. The subsequent enhancement of 

 adaptations in a given environment invaded 

 by a preadapted organism is termed post- 

 adaptation (Simpson, 1944, p. 186). 



Preadaptation through genetic modifica- 

 tion of previous adaptations may result in 

 the invasion of a new biotic habitat. Culti- 

 vated varieties of wheat have been bred 

 for their resistance to infection by rusts 

 (Chester, 1942). The variety known as 

 Kanred wheat was found to be resistant to 

 stem rust {Piiccinia graminis) , and in 1924 

 over four million acres of this variety were 

 planted in the central United States. It be- 



