J. W. H. Harrison 265 



Nor Ih it t<» ho HUp|)«imMl llmt all <»f* thr hi*tvrnzy>;<»t<*H iin* h«»t/<'rnzy^<»iiH 

 Ut the wiiiio (U'^HM', for that in far fnmi Iwin^ h«». Thry pnKMMMJ fmiii 

 inunohybridH. throu^rh every HUi^e of inatnK'linouH ami |MilnK'liiinuH coni- 

 plexe8, right up to the full heU«ro/.ygouH coinphiiu'iit of thr /', frat«Tnity. 



Hence the stimulating iinpuUe of het«Tozyg«».sity in f<'lt with uni<|ual 

 foroo in the diffep'nt zygoti»8, luul they attain vastly dirtrn-nt h'\/a'h. 

 Thus, as in characteristics so in size, the F^ generation shows cnnrrnous 

 variation as coinjMiretl with the conijNiratively even size and ap|M*anince 

 of the Fi lots. This state of atVaii*s wjis displayed vividly in the F^ and 

 Ft generations of the pomoiiarid-isabellac crosses. 



Manifestly therefore, by the men* stimulus of tln'ir heterozygosis, 

 independently of any action of multiple genes for size and weight, th«*n' 

 will lx» ap))aR>nt size segregation in the F^ insects. Any att«'mpted 

 genetic analyses for size purjxjses which fail U> allow for heterozygotic 

 impulses are vitiated and useless. 



To illustnite by examples, size and weight exjK*rimeiits have been 

 undertaken with guinea-pigs, poultry, rabbits, etc. in several of thes<\ 

 dwarf strains of sjx'cial races have been mated with full-sized examples 

 of other breeds, thereby bringing into play unknown numbei-s of genes 

 resulting in the production of an equally uncertain number of hetero- 

 zygotic composites in the F.^ animals carrying with them undetermined 

 and indeterminable degrees of size acceleration. How can it be p<j.ssible 

 with this unknown factor interfering to conduct any analysis of size and 

 weight determiners ? — hence my opinion, expressed above, that many 

 of these analyses are useless. They are not only valueless because 

 heterozygosis will produce size groups of its own, but also because its 

 effects, added to those of the size and weight determiners themselves, 

 serve to throw accelerated specimens into groups to which on the merits 

 of possible weight and size factors they do not belong. 



