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 fuscus, 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 
