MENDELISM GENERAL DISCUSSION 73 



Some interesting evidence in a case of this kind has 

 been obtained by East, by means of statistical methods. 

 In one of East's experiments, two varieties of maize 

 were crossed which measured, on the average, 68 and 

 101 inches in height respectively. This may appear 

 a definite enough difference, but all intermediate heights 

 between these two varieties can be found, and it is 

 impossible to trace any simple kind of Mendelian in- 

 heritance of the characters. In the first hybrid genera- 

 tion the average height was 95 inches, and the coefficient 

 of variation was 8'7 per cent. In the second hybrid 

 generation the average height was 82'5 inches, and 

 the coefficient of variation 13'4 per cent. Variability, 

 then, was found to be considerably greater in the second 

 than in the first hybrid generation. This is strong 

 evidence that segregation had occurred. 



We know too little as yet to be able to say definitely 

 whether Mendel's law applies with regard to all char- 

 acters, in all kinds of living organisms. It is certain 

 that it applies to various characters in many species 

 of plants and animals, and it seems doubtful whether 

 any real exception has yet been found. 



A secondary point of much interest and great prac- 

 tical importance has lately been discovered by Shull 

 and East, both working largely on maize. It is to the 

 effect that the vigour of a plant is influenced, not only 

 by the characters which it possesses, but also according 

 to whether these characters are present in the pure or 

 in the hybrid condition. 



It has long been known that inbreeding, or the 

 mating of closely related individuals amongst animals, 

 is very detrimental to the general physiological vigour 

 and the fertility of the breed. Similarly in plants where 

 cross-fertilisation is the rule, enforced self-fertilisation 

 produces much less vigorous offspring than the normal 

 method of reproduction. 



It has been found, however, that if maize, which is 

 normally cross-pollinated, be artifically self-fertilised 

 through several generations, the loss of vigour does not 

 continue at the same rate. It is large between the 



