56 Zygotes are Double Structures [CH. n 



applicable to two physiologically distinct classes of pheno- 

 mena. It was anticipated that some characters, or possibly 

 even some forms of life, might follow the one system and 

 others the other. 



The results of further researches make this supposition 

 increasingly improbable ; and though undoubtedly there 

 are cases which cannot yet be subjected to Mendelian 

 analysis, it is fairly certain that there is no large group of 

 facts in heredity to which the Galtonian system or any 

 modification of it exclusively applies. 



There are however numerous examples where the arith- 

 metical results predicable by either system are nearly or 

 quite the same, though further breeding would of course 

 reveal that even in these cases the applicability of the 

 Galtonian method was only superficial. 



The first aim of genetics must now be to determine the 

 magnitude, number and ultimately the nature of those units 

 which together make up the visible fact we call heredity; 

 and so to discover the consequences of their several com- 

 binations in zygosis or fertilisation. For the power thus to 

 formulate our purpose and for the development of a method 

 by which it may be successfully pursued we are beholden 

 to Mendel's genius. 



The difficulty which some feel in realising the signifi- 

 cance of Mendelism arises from the habit of looking on the 

 bodies of animals and plants as single structures. So soon 

 as the mind becomes thoroughly accustomed to the fact 

 that all individuals, at least those of the higher and more 

 familiar types, are double, it becomes easy to think in 

 Mendelian terms, and the world of gametes, whose pairings 

 have brought into existence the individuals we see, comes 

 naturally and persistently before the mind. Henceforth we 

 have to penetrate behind the visible appearances of the 

 individual, and endeavour to reconstruct first those pro- 

 cesses of cell-division which produced the germ-cells or 

 gametes, distributing the characters or factors among them 

 according to definite systems ; and then the subsequent 

 process of union of those gametes pair by pair, in fertilisa- 

 tion to form zygotes, each developing and manifesting in 

 its development those properties of structure, instinct and 

 conduct conferred upon it by that particular complement 

 of factors which its two original gametes contained. 



