J. W. H. IIahhison -203 



t^ HtH' ln»w th<»n» should Imvi* iiriMiii this pnntveji!niH> stiintilation IxTnum*. 

 when all is sjiid and cinni', all tin* s|Mrit'.s ••mployrd an* <li\ IiIimI hy pn*- 

 cistdy X\\v sani»» nunilHT of j^onrnitinns fnmi ih«' main l.tjcin st<K'k. It 

 musts tluMvfoiv. Ik» flin^'tly <lf|Hnd«'nl «ui tin- runmlativ.- ditVrn'ncrs 

 U»twtH*n xhv raot4»rs building up lln' various ^mutyprs (or iMntyjH-.s, it 

 preforrtsl !). 



But. how ha\M' tht's«* s|M»cit's ;iiis«'n .-nKl ln>\v {\n tlnv ditVrr ' Wlutln-r 

 thoy have Ihm'U t'volvrd by Ljfnirraphical isolatinii (.r by mutation th«'ir 

 dit^'eivnco tVom tlu'ir innn<'diat<> phyl<>i^('n<'t ic ancestor ap|H'an'd jus a 

 change in thr valuo of th«> t^i-nrs ritlwr by loss, addition, dujilication or 

 othonviso ; wh«'ntH* it is i'vi<l»'nt, that.. ;is wr pass from the Lycia stem to 

 the furthest rt'nu»ve<l tbnn (physiologically and |)hylo^eii(ti(alIy ), wr 

 have. ;ki/*i* passu, accompanying ilirterences iu what wen- originally 

 homolog«)us genes plus the apjM'anince of genes not re|)n'sented in the 

 original stock. Therefore, if we |),iir A. Inrtavin with any <»f its (h-rived 

 fonns, we i\w geneniting zygotes' the <legree of h<'terozygosity of which 

 depends entirely- ujx>n the extent of the <liv<'rgence of th«' second form 

 from hirtaria. And this corollary must follow then-from, that the in- 

 cre;u<i'd and progressive met^ibolic action visible in the hybrid series is 

 directly and positively correlated with the advance in the heterozygosity 

 of the individual hybrid membei*s. 



Nix?ess;irily, of couinc, a pr>int must be reached when, in the end, 

 total or pjirtial incompatibility^ of the two setsof chromosomes involved 

 will step in c^irrying with it, in some, physiological interference with 

 the normal development of the zygote and, in others, the total failure 

 of fertilisation ; thus limits are set to the heterozygotic acceleration of 

 the activities of an organism by means of hybridity. 



As to how the influence of the heterozygosity works there are 

 several possibilities open. Firstly, one must take cognisiince of the 

 fact that any intruding spermatozoon consists of little but the nuclear 

 structures which, coming into intimate contact with the cytophusm 

 of the ovum, are so placed as to be able to act, react, and interact 

 with that cytophusm which presents it with entirely novel conditions of 

 environment ; thus the stimulus may be brought about. Secondly, the 

 heterogeneous nature of the zygote, the conseijuent conflicting tendencies 

 of its genes, the extra work induced by the interaction, all combine t<» 

 secure the laying down of great stocks of cytoplasm and stimuiat** to 

 an extraordinary degree cell-division, not only in the actual number of 

 cells, but in the sj)eed with which they are elaborated. On this view, 

 * As when Lycia hirtaria j and PorcilopKix nicfwliie , are mated. 



