104 



THE RATE OF GROWTH 



[CH. 



' In a very well-known paper, Bateson shewed that, among a 

 large number of earwigs, collected in a particular locality, the 

 males fell into two groups, characterised by large or by small 

 tail-forceps, with very few instances of intermediate magnitude. 

 This distribution into two groups, according to magnitude, is 

 illustrated in the accompanying diagram (Fig. 23) ; and the 

 phenomenon was described, and has been often quoted, as one 

 of dimorphism, or discontinuous variation. In this diagram the 

 time-element does not appear ; but it is certain, and evident, that 

 it lies close behind. Suppose we take some organism which is 



T3 100 



Length of tail-forceps, in mm. 



Fig. 23. Variability of length of tail-forceps in a sample of Earwigs. 

 (After Bateson, P. Z. S. 1892, p. 588.) 



born not at all times of the year (as man is) but at some one 

 particular season (for instance a fish), then any random sample 

 will consist of individuals whose ages, and therefore whose magni- 

 tudes, will form a discontinuous series ; and by plotting these 

 magnitudes on a curve in relation to the number of individuals 

 of each particular magnitude, we obtain a curve such as that 

 shewn in Fig. 24, the first practical use of which is to enable us 

 to analyse our sample into its constituent "age-groups," or in 

 •other words to determine approximately the age, or ages of the 

 fish. And if, instead of measuring the whole length of our fish, 

 we had confined ourselves to particular parts, such as head, or 



