212 



THE RATE OF GROWTH 



[CH. 



limited. A linear relation, or simple-interest law, seems less likely 

 to occur; but the fact is, it does occur, and occurs commonly. 



On so-called dimorphism 



In a well-known paper, Bateson and Brindley 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 



DD 



Fig. 57. Tail-forceps of earwig. From Martin Burr, after Willi Kuhl. 

 150 



^ 100 



mm. 10 



Length in mm. 



Fig. .58. Variability of length of tail-forceps in a sample of earwigs. 



After Bateson and Brindley, P.Z.S. 1892, p. 588. 



tail-forceps (Fig. 57), with few instances of intermediate magnitude*. 

 This distribution into two groups, according to magnitude, is 

 illustrated in the accompanying diagram (Fig. 58); and the 



* W. Bateson and H. H. Brindley, On some cases of variation in secondary 

 sexual characters [Forficula, Xylotrupa], statistically examined, P.Z.S. 1892, 

 pp. 585-594. Cf. D. M. Diakonow, On dimorphic variability of Forficula, Joum. 

 Genet, xv, pp. 201-232, 1925; and Julian Huxley, The bimodal cephalic horn of 

 Xylotrupa, ibid, xviii, pp. 45-53, 1927. 



