ALLEN: EVOLUTION OF SEXUAL CHARACTERS 11 



ancestry, certain of which have been inhibited, but not completely suppressed, 

 by mutations like those previously mentioned. 



But apart from variability of this common type, now and then a demon- 

 strable mutation occurs in the direction from dioecism or monoecism toward 

 hermaphroditism. Such mutations are known in dioecious species of Lychnis, 

 Sali.r, Silene, Vitis, and Fragaria; in Salix and Silene they have occurred 

 in the offspring of interspecific crosses. One recessive mutation resulting 

 in bisexual flowers is known in maize (7). Most monoecious or herma- 

 phroditic strains derived from dioecious species and subjected to genetic 

 experiment have behaved as though they were mutated males ; a very few 

 ha^-e seemed to be mutated females. In Lychnis both mutated males and 

 mutated females have been recognized cytologically. Those hermaphrodites 

 (the term is often somewhat loosely used) which appear to be mutated males 

 in general behave in breeding like males ; that is, their progeny shows them 

 to be heterozygous for a sex-tendency gene. In this respect they differ from 

 regularly hermaphroditic species, which of course transmit hermaphroditism 

 uniformly to all their progeny. This genotypic difference, as Correns pointed 

 out, justifies the description of the appearance of hermaphroditism in a 

 dioecious or monoecious species as a case of "backward evolution." The implica- 

 tion is that in a dioecious species derived from a primitively hermaphroditic 

 one a mutation has produced a reversion to the phenotypically original char- 

 acter — although this change is not due to a reverse mutation of a previousl}' 

 mutated gene. 



Xot always readily distinguishable from these variations and mutations 

 are the reported cases, in species classed as dioecious, of strains which regu- 

 larly vary in degree of sex-separation. In Urtica cannabina, Spinacia, and 

 Mcrcitrialis, for example, plants shown to be genetically distinct occur which 

 are monoecious or hermaphroditic. In other instances differing degrees of 

 sex-separation are manifested by different strains. Comparabk but not fully 

 elucidated cases are presented by gynodioecious species. In the absence of 

 direct evidence as to their origin, these diverse conditions are capable of 

 explanation either as steps in an evolutionary sequence leading toward 

 dioecism, individuals showing intermediate conditions not yet having been 

 eliminated ; or as evidences of mutation in the reverse direction comparable 

 wdth the cases studied in Lyclinis. 



]\Iutations, then, may and do occur both in the general direction from 

 hermaphroditism toward dioecism and in that from dioecism toward herma- 

 phroditism. Those of the latter class are much the less frequent, and the 

 best known of them lead to a hermaphroditism which is not genetically like 

 the hermaphroditism which may be considered primitive. It is evident, too, 

 Ihat mutations away from hermaphroditism have been numerous in the past 



