204 Plant Genetics 



scheme ? Two attempts to devise such a unifying con- 

 ception will be presented briefly for what they are worth. 

 Both are highly theoretical. 



The first of these theories is rather simple, sketching 

 a picture which is easy to visualize as a whole but is 

 inexact in its detail. Sex determination may be taken 

 as an example of the more general situation. The deter- 

 mination of sex may be likened to a pair of balances. If 

 one side goes down a male is the result; if the other side 

 goes down a female is the result. What are the weights 

 that operate the balance ? They are of two kinds; one 

 is represented by the sex chromosomes, the other by 

 the physiological conditions. The sex chromosomes are 

 very heavy weights, so that if they are put on one side of 

 the balance, it would require a great many physiological 

 weights on the other side to counterbalance them. 

 Nevertheless, this can be done. With the heavy chro- 

 mosome weights all on one side they can still be over- 

 balanced by piling all of the physiological weights on 

 the other. In the organisms in which the sex chromo- 

 some theory seems rigidly estabUshed, it means that the 

 chromosome weights are relatively heavy and caimot 

 be counterbalanced by all of the physiological weights 

 on the other side; or it may merely mean that all of the 

 physiological weights have not yet been discovered. 

 In other forms the chromosome weights are not so heavy, 

 and can be overbalanced by the known physiological 

 weights.' 



It may be asked why the chromosome weights are 

 imagined to be so much heavier in some forms than in 

 others. The explanation may be that where the chromo- 



'This analogy of weights has been suggested by Doncaster (i). 



