CORRELATION 171 



mutation occurs. The degree of correlation will depend on 

 the susceptibility of the characters to environmental influences. 

 Again, the phenotypic expressions of dominant genes lying in 

 the same chromosome will be more or less strongly correlated, 

 depending on the amount of crossing over. We can even 

 invent a hypothetical case in which two characters would 

 show complete correlation, by assuming that each of the genes 

 responsible was lethal when not associated with the other. 



On the whole we believe that the bulk of intraspecific 

 correlations is due to most members of a species being homo- 

 zygous for their distinctive characters. As Fisher (1930, p. 124) 

 has said, ' the intimate manner in which the whole body 

 of individuals of a single species are bound together by 

 sexual reproduction has been lost sight of by some writers. 

 Apart from the intervention of geographical barriers so 

 recently that the races separated are not yet regarded as 

 specifically distinct, the ancestry of each single individual, if 

 carried back only a few hundred generations, must embrace 

 practically all of the earlier period who have contributed 

 appreciably to the ancestry of the present population. If we 

 carry the survey back for 200, 1,000 or 10,000 generations, 

 which are relatively short periods in the history of most species, 

 it is evident that the community of ancestry must be even 

 more complete. The genetical identity in the majority of 

 loci, which underlies the genetic variability presented by most 

 species, seems to supply the systematist with the true basis of 

 his concept of specific identity or diversity.' Hagedoorn and 

 Hagedoorn (1921) have expressed the same idea in a rather 

 different way. In nearly all species the population is not of 

 constant size throughout the year or from one year to the next. 

 This is particularly obvious in all species which, in temperate 

 climates, have a definite breeding season. The large popula- 

 tion existing at the end of the breeding period is gradually 

 depleted till only a relatively small number is available to 

 breed again the next year. The survival of only a small 

 number to carry on the species must mean an enormous 

 reduction in variation each year, probably enough to account 

 for the observed constancy of most species. The chance that 

 any variant represented by only a few individuals will form a 

 part of the next year's initial population is very low, the 

 magnitude of the chance depending (apart from survival 



