CORRELATION 



163 



correlation between characters diagnostic of pairs of related 

 species. 



It will be seen that in these examples, which have been 

 collected at random, the average correlation is o • 56, which is 

 a fairly high figure. 



In all probability this figure is rather in excess of the general 

 average. Thus, in his analysis of the variation of various 

 groups of invertebrate fossils, Trueman (I.e.) emphasises the 

 tendency for the characters of species to vary independently 

 of each other, and the consequent low correlation. 



(b) Alkins (1928) has analysed the variation of the land 

 snails Clausilia rugosa and C. cravenensis in a study which is 

 particularly valuable on account of its being based on samples 

 taken from different colonies (though from a restricted region). 

 He studied two of the diagnostic characters, viz. length and 

 major width of the shell. He gives no statement of the corre- 

 lation of those characters in the two species over the whole 



