56 



VISIBLE CHARACTERS SHOWN IN SECOND GENERATION HYBRIDS. 



A B A b a B a b 



Crossing No. /Number 221 78 71 25 



I. \ Relation 9 : 3-2 : 2-9 : 1 



Crossing No. /Number 250 87 82 33 



II. '\Relation 9 : 3-1 : 3-0 : 1-2 



Crossing No. /Number 1342 445 513 168 



III. \Relation 9 : 3-0 : 3-4 : 1-1 



The above proportions, it will be noted, correspond very closely with 

 what might be theoretically expected, viz., the proportions 9:3:3:1. 



As has already been pointed out in this paper, the number of possible 

 combinations is increased immensely, the greater the number of differen- 

 tiating characters involved in the crossing. The proportions, in normal cases 

 at least, agree with the same law as that which governs the union in mono- 

 hybrid and dihybrid combinations. 



From the above elucidation of the Law of Mendel there emerges the 

 two essential discoveries of that law, one of which indeed may be said to be 

 complementary of the other. These are as follow: 



(a) The different characters behave as units and, during the process of 

 reproduction, segregate and are carried over from one generation to another 

 without undergoing any essential change. 



(b) The different characters act independently of each other by reason 

 of which fact many different combinations may be effected by different 

 groupings of a relatively small number of units. 



Thus has the Mendelian annunciation thrown an entirely new light on 

 the nature of hereditary variations and has introduced quite a new principle 

 into biological science. 



In establishing his law of hybrids, Mendel wisely enough worked \vith 

 simple differentiating characters, and it was upon the behaviour of these 

 that he framed his law. At the same time he premised that complications 

 would doubtless occur which would require a further exploitation of the 

 principles involved to explain. These complications have come and have 

 been met by various workers. To some they have been discouraging and 

 confusing; to others they have served as a stimulant to further investiga- 

 tion and study. Crossing work in great extension has therefore been prose- 

 cuted and the progeny studied with extreme care. In this way the unit 

 constitution of many sorts used as parents has been determined. This 

 knowledge has served not only to elucidate some of the apparent irregu- 

 larities in, or exceptions to, the Mendelian annunciation, but also seems to 

 offer an explanation of the appearance of many of the strange forms in our 

 cultivated crops which indeed is our chief concern just now. 



