202 MENDELISM 
only just commenced, and it is not unlikely that 
additional members Of the series still remain to be 
discovered. 
The converse of coupling is sometimes shown in 
what is known as gametic repulsion. Thus certain 
F 2 ratios have been observed which are most readily 
interpreted on the supposition that a particular mem- 
ber of one pair of allelomorphs is unable to exist in 
the same gamete with a particular member of a 
distinct pair. 
The second class of complications that we have to 
deal with—although the term complication may be 
to a certain extent justified in connection with it— 
does not involve any exception to Mendel’s law of 
segregation. The phenomenon of so-called reversion 
on crossing has long been familiar to biologists. Its 
meaning, however, was totally obscure, and even the 
Mendelian was at first unable to offer any explanation. 
The phenomenon consists in the appearance, in the 
offspring of a cross, of a character which was not 
visibly present in either parent, and in many cases this 
character can properly be regarded as ancestral—it is 
a character which has been lost by both parents in 
the course of their divergent evolution from a common 
primitive form. Now, these cases differ entirely from 
those of the appearance of a heterozygote form on 
crossing, such as are due to the combined action of the 
two parental allelomorphs in the cross-bred offspring, 
because in true cases of reversion a certain proportion 
of the reversionary individuals of F, are found to breed 
true, which a simple heterozygote will never do. 
