THE CATEGORIES OF VARIANT INDIVIDUALS 75 



habits, etc., lead to structural change ? The demonstration 

 by Nuttall (19 14), Bacot (191 7) and Sikora (191 7) that the 

 human head-louse could be transformed into the body-louse 

 by transference from the head to the arm is interesting in 

 this connection. 



The physiological race presents no special difficulty in 

 our scheme of categories. How far they are (a) regularly 

 distinguished as discontinuous populations and (b) hereditarily 

 fixed are more difficult questions, and there are not sufficient 

 data to answer them. Races habituated, e.g., to different 

 food-plants will obviously be dis- 

 continuous, but some contrasted 

 types of habitat-preference are 

 certainly not. As regards the 

 hereditary fixation of such racial 

 characteristics little can be said 

 at present. The experiments with 

 Pediculus [anted) and Thorpe's ex- 

 periments (1929) with Hyponomeuta " pitn ? 

 seem to suggest that physiological 

 preferences are not germinally fixed. 

 Harrison's claim to have induced 

 a new germinally fixed habit of 

 oviposition in Pontania (1927), in- 

 volving the acquisition of a pre- 

 ference for a new host-plant, does 

 not seem to be justified (see p. 41). 



Pi/estimen/i 6 



P.vcstiminh' ^ 



Fig. 4. — Body-Lice (larger 

 specimens) and Head- 

 Lice {Pediculus). 



(From Sikora, 191 7.) 



Different methods of analysing the variation of natural 

 populations have shown that it is not without order and the 

 most obvious tendency is for individual variants to form groups 

 of various kinds. These groups are aggregates of individuals 

 resembling each other usually in a number of correlated 

 characters. The simplest and most fundamental manifestation 

 of this tendency is seen in the homogeneous stocks produced 

 by vegetative or autogamous reproduction. The mechanism 

 of heredity produces another kind of group in the biotype and 

 combined with autogamous reproduction, the pure line. A 

 third kind is produced by topographical and other barriers 

 to intercourse, and here it is customary to indicate the degree 

 of divergence by a hierarchy of grades beginning at the colony 



