An Introduction to a Biology 



biologist, the chances are that he will be inclined to ask me, 

 !< What does it matter what the length-by-breadth index of 

 a duck's bill is ? " My answer to this interruption is that 

 so long as we are as much in the dark as we are at present 

 about the circumstances which may affect an animal's or 

 plant's chances of attaining maturity, any statement that 

 such and such a feature matters or does not matter is un- 

 warrantable. 



But let us return to our argument. Some living things 

 are variable. We may adopt two attitudes towards this 

 variability. We may either say that it does not matter 

 and ignore it, or we may suspect that it may matter and 

 measure it. In my opinion, evidence does not justify us 

 in adopting the former attitude. Statisticians have pro- 

 vided us with a method for measuring this variability. But 

 we usually want to know more than this ; we want, if pos- 

 sible, to measure the closeness of the relation between two 

 such variable things. Statisticians have again provided us 

 with a method which enables us to measure the closeness of 

 that relation in which biologists are most interested, namely, 

 that between parents and children. 



The first step in this method is to construct a Correla- 

 tion Table. How this is done is best explained by giving an 

 account of Weldon's beautiful experiment. The variable 

 phenomenon he dealt with was the number of dice, in a 

 throw of 12, which fell so that faces with 4-or-more pips 

 on them were uppermost. When we throw a single die it is 

 an even chance whether it falls so that a face with 3-or- 

 fewer pips on it lies uppermost, or whether a 4-or-more- 

 bearing face lies uppermost. Therefore the most probable 

 number of dice with faces bearing 4-or-more uppermost in 

 a throw of 12 is 6, but the number may be anything between 



and 12 inclusive, though these extreme results occur very 

 seldom. Here is a list showing the frequency with which 

 the 13 possibilities occurred in a thousand throws which 



1 have made : 



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