INTERPRETATION OF REVERSION 117 



that purple spot was not dependent on the simul- 

 taneous presence of grey for its manifestation. 

 But on the supposition that the purple spotting 

 cannot exist in the absence of grey, the seed-coats of 

 these three plants will be simply white, and, there- 

 fore, indistinguishable from the single white plant 

 occurring at the end of the series, in the ratio of one 

 in the whole sixteen. The first 3 in the series will 

 not be affected by this supposition, because grey can 

 exist in the absence of purple spot ; nor will the 9, 

 because both characters are present. The second 

 hybrid generation will, therefore, consist, according 

 to this theory, of nine plants the seed-coats of which 

 are grey with purple spot, three grey and four white. 

 And this, it will be remembered, is the result which 

 was actually obtained. Of course, the fact that the 

 theory harmonises with the facts does not prove that 

 the theory is true. Its truth can only be tested by 

 finding out whether all the consequences which follow 

 from it actually occur. 



But assuming that it is true — and I, for one, believe 

 that it approximates fairly closely to the truth — it 

 brings us within reach of an explanation of one of 

 the phenomena with which the breeder is most 

 familiar, namely, that of reversion, or throwing back. 

 Reversion on crossing, in such cases as those which 

 we have been considering, is due, according to this 

 theory, to the reunion in one individual of two 

 characters, the simultaneous presence of both of 

 which is necessary for the existence of the ancestral 

 character. In the instance discussed above, the 



