ECOLOGY AND GENETIC VARIATION 



601 



"the smaller and more distant an island is 

 from the nearest mainland, . . . the fewer 

 species will be found." New Guinea has 509 

 species of native land birds; the Solomon Is- 

 lands, 138; the Fiji Islands, 54; Samoa, 

 33; the Society Islands, 17; the Marquesas 

 Islands, II; Henderson Island, 4; and Eas- 

 ter Island, none. The number of habitats 

 as well as the degree of isolation is doubt- 

 less reflected in these data. 



We may now consider a few cases in 

 which environmentally induced mutation 

 exhibits a response adapted to the inducing 

 agent. In the organisms in which there is 

 Uttle distinction between somatic and ger- 

 minal tissue or between cytoplasmic and 

 nuclear protoplasms, direct modification of 

 the protoplasm, if autocatalysis occurs, 

 may result in particular hereditary changes 

 directly adjusted to the environmental in- 

 ductor (Sturtevant, 1944). If a gene pro- 

 duces an antigen, the antibodies induced 

 by this antigen might react with the gene 

 as well as with the antigen (S. Emerson, 

 1944). This would be a form of induced 

 mutation with the response functionally 

 related to the stimulus. 



An example of a hereditary response to 

 the inducing agent is found in the experi- 

 ments upon pneumococci. If placed in un- 

 favorable physical, biochemical, or nutri- 

 tional conditions, reactive phases of thirty- 

 odd known serological types of virulent 

 pneumococci {Diplococcus pneumoniae) 

 change in virulence, in their abiUty to form 

 capsules, in immunological characteristics, 

 and in colony configuration. Some of these 

 changes are of a dissociative or degenera- 

 tive nature in which the resultant degraded 

 form loses the antigenic character of its 

 special type (of polysaccharide haptene), 

 but retains the antigenic action common to 

 the various serological types of the species. 

 The original type may be produced from 

 the degenerate form by passing the strain 

 through a susceptible animal, by growing 

 the colony in a medium containing an an- 

 tiserum produced by immunization with the 

 degenerate forms, or by adding heat- 

 killed virulent cultures of the original 

 type to the medium growing the degener- 

 ate form. If heat-killed virulent cultures of 

 a different type are placed with the degen- 

 erate form, it may regenerate into a heredi- 

 tarily stable type corresponding to the 



source of the heat-killed virulent culture 

 (White, 1938, p. 117; Morgan, 1944). 



The active substance producing such 

 transformations has been isolated and iden- 

 tified in an extract of Type III pneu- 

 mococci (Avery, MacLeod, and McCarty, 

 1944). The percentages of carbon (34 to 

 35), hydrogen (3.7 to 3.8), nitrogen (14 

 to 15), and phosphorus (8.5 to 9.0) agree 

 closely with those calculated for sodium 

 desoxyribonucleate (tetranucleotide). The 

 fact that this type of nucleic acid is found 

 in nuclei and chromosomes of higher or- 

 ganisms lends credence to the theory that 

 the genetic materials in the pneumococci 

 may be affected directly (Morgan, 1944). 



In most organisms the germ plasm is 

 buffered by the soma, and the nucleus is 

 buffered by the cytoplasm; but in the bac- 

 teria and a few other organisms, the ex- 

 ternal environment may be in closer phys- 

 ical or chemical contact with the germinal 

 mechanism (Hinshelwood, 1944). 



It should also be realized that selection 

 acting upon billions of micro-organisms un- 

 dergoing rapid reproduction may play a 

 guiding role within a few days commen- 

 surate with the action of selection over a 

 period of many centuries in slowly repro- 

 ducing higher organisms. Some of the adap- 

 tations of bacteria to new media may be 

 the result of selective elimination (Lewis, 

 1934; Braun, 1945; Severens and Tanner, 

 1945; see also pp. 628, 629), but in the 

 pneumococci the environment seems to 

 have a direct effect. Sonneborn (1943) 

 points out that the transformation of the 

 antigenic type of Pneumococcus by reduc- 

 tion to a nonspecific form, and treatment 

 wdth heat-killed cells of another type, may 

 be equivalent to removing one factor and 

 substituting another that then activates an- 

 other gene. In view of the biochemical in- 

 vestigations of Avery, McCleod, and Mc- 

 Carty (1944), it may even be possible that 

 characteristic nuclear material itself is sub- 

 stituted. 



It would appear that both genes and cy- 

 toplasm may be required for the expression 

 of an inherited character over a long ser- 

 ies of generations (Sonneborn, 1942). 

 When certain cytoplasmic substances are 

 initially present, the gene may stimulate 

 their further production, but cannot pro- 

 duce the substance in the absence of the 



