D.— ZOOLOGY 109 



those of the ages associated with the fullest expression of the reproductive 

 function, and that through selection all the mechanisms, whatever they 

 may be, which influence this are fashioned into harmony with this " re- 

 productive " sex ratio. It is necessary, therefore, to discuss the meaning 

 in biological philosophy of mutation, sex and heterogamety, all of which 

 are intimately concerned. 



Mutation is a mechanism that has become adapted and elaborated for 

 the provision of material possibilities of evolutionary movement. It takes 

 the form of the replacement of one gene by an allelomorph : — the sub- 

 stitution of a gene that, having been tested and found worthy, has come to 

 be in harmony with the rest of the genotype, and through this with the 

 conditions of the external environment, by another, the merits of which 

 have yet to be determined. More often than not, therefore, mutation 

 implies a disturbance of an equilibrium within the genotype itself, and 

 between this and the external environment. It is usual, therefore, to 

 find that mutation leaves the genotype less in harmony with the existing 

 external conditions than was that which it has replaced. Mutation, the 

 inception of a new heritable variation, thus usually equips its possessors 

 with a handicap, imposing on them a disadvantage that can range from 

 inconvenience to complete lethality. 



Sex is the mechanism that has become adapted for the dispersal of a 

 mutant gene amongst a population. Through its exercise a new gene 

 can become incorporated into a variety of genotypes, and thus given the 

 opportunity of finding itself associated with other genes with which it 

 can interact. It is established that the quality of the action of a gene is 

 largely determined by this interaction with others, for, as judged by its 

 effects upon the life processes of the zygote, it can exhibit disadvantageous 

 properties when in one gene association, advantageous when in another. 

 For the quiet dispersal of a mutant gene in this way it is necessary that it 

 should be recessive, initially at least, to the gene that it has displaced, for 

 then it can pursue a cryptomeric existence for a considerable period of 

 time. 



If the great usefulness of sexuality lies in the opportunities it provides 

 for variety in gene recombination and for the exercise of selection then its 

 advantages will be best exploited when, among those about to reproduce, 

 there is a sex ratio of equality. This observation would not seem to apply 

 to those instances in which gene combinations that are common or are 

 exceedingly rare are concerned, but a sex ratio of equality among the 

 parents would seem to provide the greatest variety of recombinations in 

 the oflFspring in the case of gene combination of intermediate rarity. Be 

 this as it may, a device for the production of a sex ratio of equality has 

 certainly been elaborated. 



Heterogamety is a mechanism that has become adapted for the pro- 

 duction of a primary sex ratio of equality. In its evolution in the higher 

 forms it has taken the form of the replacement in one sex of an X-chro- 

 mosome by a wholly or largely non-homologous Y-chromosome. The 

 heterogametic sex, therefore, now possesses one X-chromosome, the 

 homogametic, two. An indirect result of this evolution has been that 

 a recessive mutant gene in the differential segment of the X-chromosome 



