168 Plant-Breeding 



mentally a mere game of chance, but follow a law of 

 regularity of averages ; but the results are so often masked 

 that it is sometimes impossible to recognize the law. 



It is a question, of course, whether the proportional 

 results secured by Mendel and others express a biological 

 principle, or whether they are only the numerical propor- 

 tions that may be adduced from the averages of large 

 numbers of combinations whether these combinations 

 are of gametes or letters, or words, or figures. It is a 

 fundamental necessity that certain proportions follow 

 from " chance " combinations often repeated. But whether 

 the " theorem of probabilities" can express a real bio- 

 logical fact may well be doubted. Perhaps the basis of 

 heredity is something more than the mechanico-physical 

 conceptions that we habitually apply to it. 



Mendel's law of heredity is stated as follows by Bateson 

 and Saunders : "The essential part of the discovery is the 

 evidence that the germ-cells or gametes produced by 

 cross-bred organisms may in respect of given characters 

 be of the pure parental types and consequently incapable 

 of transmitting the opposite character; that when such 

 pure similar gametes of opposite sexes are united together 

 in fertilization, the individuals so formed and their pos- 

 terity are free from all taint of the cross, that there may 

 be, in short, perfect or almost perfect discontinuity 

 between these germs in respect of one of each pair of op- 

 posite characters." 



The genetic constitutions of plants, if they are known, 

 may be conveniently represented by formulae containing 

 the gametic make-up of the parents entering into their 

 union. At least such unit-characters as are known may 



