CHAPTEK III. 



COKEELATED VARIATIONS. 



The measurement of correlation Gallon's function Correlation 

 between various organs in man, in local races of the shrimp, and in 

 crabs Comparison between primitive and civilised races of man 

 Correlation between morphological characters and the reproduct- 

 ive system Genetic Selection in man Especial fertility of type 

 forms in certain plants Evolution in the Peppered moth Paral- 

 lel variation Importance of mathematical treatment of variation. 



ALL parts of an organism are to a certain extent re- 

 lated to each other, so that when one part varies other 

 parts vary simultaneously in a greater or less degree. 

 That is to say, variations are correlated. The most 

 marked and obvious correlation is that existing between 

 homologous parts. The symmetry of the correspond- 

 ing or homologous organs on the right and left sides of 

 the body, which is present in most animals, represents 

 a very close degree of correlation. But even in this 

 case the correlation is not constant or complete. Thus 

 the two arms and the two legs of a man resemble each 

 other very closely indeed, but careful measurement 

 shows that the resemblance is not absolute. Again, the 

 arms, as a rule, vary in length more or less in the same 

 proportion as the legs, but personal experience will 

 probably recall instances to the contrary, in which the 

 length of the limbs was quite disproportionate. Be- 

 tween the arms and the legs, therefore, the degree of 



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