W. F. R. Weldon 287 



presence of u jiarlicular iiiüt clcniriiti in llic ijcrm-ci'll ; and lliis ciinccpt ion u\' thc 

 " mechanisni " of iiiheritancu eiiablud him to foriuulate his hypothesis of tbc Con- 

 stitution of thc germ-cells in the ulass of hybrids which he studied. He believed 

 that if two races dififer in a paiticular uuit-eharacter, which had the condition A 

 in one race, the condition a in another, the gatnetcs of the two races will differ; 

 in the one race, each gamete produced will contain an elcmcnt A, each ganiete of 

 the other race coutaining an eleinent ((. In thu normal process of reprodiiction by 

 individuals of like race, male and f'emalu gametes of likc Constitution mect, and 

 their corresponding elements unite ; so that the complete Zygote f'roni which a 

 pure-bred individual of cur first race arises, contains an elenient AA, deriv(.'d in 

 part from each parent ; when an individual ot'one raoe is paired with an individual 

 of the other race, the hybrid Zygote contains an dement Aa. Novv in cases such 

 as those Mendel observed, he supposes that when the hybrid, resulting from a 

 Zygote Aa forms germ-cells for itself, each of its germ-cells contains either an 

 element A only, or an elemeut a only ; and germ-cells of each kirid are formed in 

 equal numbers, half of each kind being male, and half female. When such hybrids 

 are paired together, three kinds of uniou are possible ; gametes of like Constitution 

 may unite, giving rise to zygotes of Constitution AA or aa, or gametes of unlike 

 kind may unite, and give rise to zygotes of Constitution Aa. As the union is 

 supposed to occur by chance, zygotes of Constitution Aa, will occur, in any large 

 series of cases, twice as frequently as zygotes AA or aa. In all his discussions of 

 such hybrids Mendel assumes that the Law of Dominance will hold, as he teils us 

 it always held in the cases he examined ; he therefore assumes that the first 

 formed hybrid, though containing Clements Aa, will präsent the appearance of an 

 individual belonging to the race in which the unit character has the "dominant" 

 condition, say A ; the offspring of such hybrids will externally be of two kinds; 

 one quarter resembliug the race a and capable of trausmitting only the characters 

 proper to that race, three quarters resembling the race A ; but of these three 

 quarters with dominant appearance, one will contain only elements ^4 and will, 

 therefore, be capable of trausmitting only the characters of race A, while the other 

 two are of composition Aa, and behave like their hybrid pareuts if allowed to 

 breed. 



If the two pure-bred parents difTer in more than one unit character, all the 

 elements which represent the various ditiereutial characters are separated in the 

 hybrid body during gamete-formation, and each behaves independeutly of the 

 others ; thus an individual of pure race distinguished by two unit characters A and 

 B, when paired with an individual of another pure race with characters a and b, 

 will give rise to a hybrid of Constitution Aa + Bb ; and when this hybrid forms its 

 gametes, separate elements A, a, B, b, will be formed in equal numbers and united 

 in the gametes in each of the four possible ways with equal frequency ; so that 

 there will be four equally numorous sets of gametes, of Constitution {A + B), 

 (A +b),{a + B),i\.m\ (a + b). A similar process of resolutiou and reconstruction 

 is supposed to occur in more conqjlex cases. 



