Sex as an Adaptation 421 



4. Exceptional Cases. — Occasionally in a species that is uni- 

 sexual an individual is found that is bisexual. The male of 

 the toad, Pelobates fit sens, has frequently a rudimentary ovary 

 in front of the testis. The same thing has been found in sev- 

 eral species of fish. In Serranus, a testis is present in the wall 

 of the ovary, and the eggs are said to be fertilized by the sper- 

 matozoa of the same individual. In frogs it has been occasion- 

 ally found that ovary and testis may be associated in the same 

 individual, or a testis may be present on one side, and a testis 

 with an anterior ovarian portion on the other. Cases like 

 these lead up to those in which the body itself may also show 

 a mosaic of sex-characters, and it is noticeable that when 

 this occurs there is nearly always a change in the reproduc- 

 tive organs also. Thus butterflies have been found with the 

 wings and the body of one side colored like the male and the 

 other side like the female. Similar cases have also been found 

 in bees and ants. Bees have been found with the anterior part 

 of the body of one sex and posterior part of another ! 



The preceding cases illustrate, in different ways, the fact 

 that in the same individual both kinds of reproductive organs 

 may suddenly appear, although it is the rule in such species 

 that only one set develops. Conversely, there are cases 

 known, especially amongst plants, in which individuals, that 

 usually produce male and female organs (or more strictly 

 spores of two kinds from which these organs develop), pro- 

 duce under special conditions only one or the other kind. 

 Facts like these have led to the belief that each individual is. 

 potentially bisexual, but in all unisexual forms one sex 

 predominates, and the other remains latent. This idea has 

 been the starting-point for nearly all modern theories of sex. 



An excellent illustration of this theory is found in those 

 cases in which the same individual may be male at one time 

 and female at another. For instance, it is said that in one of 

 the species of starfish (Asterina gibbosa) the individuals at 

 Roscoff are males for one or two years, and then become 



