NUMBERS : DYNAMICS 75 



time become modified by the interaction of genes 

 and selection of a gene-complex which then alters 

 the expression in characters of the originally harmful 

 or useless genes. Ford's field studies (1930) of the 

 butterfly {Melitaea) support his hypothesis. The 

 mystery of the non- adaptive differences between 

 closely allied species (that is to say the mystery of 

 the origin of species) has still to be explained, but 

 it is clear that population studies in the field and the 

 laboratory have an important part in the further 

 development of such evolution theories. A possible 

 synthesis of the views already mentioned has been 

 suggested by Elton (1931a). It is possible that 

 manj^ genes establish themselves (either in fluctuat- 

 ing or in relatively stable populations) through their 

 adaptive importance to the viability of the organism 

 during development. That is, a gene may be an 

 important physiological component of a highly viable 

 gene -family or gene -complex (see Ford, 1931), but 

 stfll produce ecologically useless morphological charac- 

 ters. ' If a boy happened to be very ill while he 

 was staying with a French family, the necessity for 

 giving unselfish attention to him, or the money 

 gained by looking after him, might make the family 

 as a whole more successful morally or financially 

 than they might otherwise have been. But this 

 would not prevent the boy growing (perhaps as a 

 result of the iUness) into a little beast or merely very 

 duU and useless ' (Elton, 1931«, 131-2). 



Fluctuations in numbers are also of importance in 

 causing alternately different types of natural selec- 

 tion, or at any rate varying their importance (Elton, 

 1924). The migration movements which are associ- 

 ated with population problems in animals also 

 probably result in bringing the animals into contact 

 with new environments and new situations, which 

 may permanently modify their evolutionary trends 

 (Elton, 1930). And finally, as was pointed out in 

 the last chapter, the study of optimum population 



