ILLUSTRATIVE OF 'NATURAL SELECTION, 159 
and, though slight, are tolerably constant in each local- 
ity. The steps, however, are so numerous and gradual 
that it would be impossible to define many of them, 
though the extreme forms are sufficiently distinct. Pa- 
_pilio Sarpedon presents somewhat similar but less nu- 
merous variations. 
4. Co-existing Variety.—This is a somewhat doubtful 
case. It is when a slight but permanent and heredi- 
tary modification of form exists in company with the 
parent or typical form, without presenting those inter- 
mediate gradations which would constitute it a case 
of simple variability. It is evidently only by direct 
evidence of the two forms breeding separately that this 
can be distinguished from dimorphism. The difficulty 
occurs in Papilio Jason, and P. Evemon, which in- 
habit the same localities, and are almost exactly alike 
in form, size, and colouration, except that the latter 
always wants a very conspicuous red spot on the under 
surface, which is found not only in P. Jason, but in all 
the allied species. It is only by breeding the two in- 
sects that it can be determined whether this is a case of 
a co-existing variety or of dimorphism. In the former 
case, however, the difference being constant and so very 
conspicuous and easily defined, I see not how we could 
escape considering it as a distinct species. A true case 
of co-existing forms would, I consider, be produced, if 
a slight variety had become fixed as a local form, and 
afterwards been brought into contact with the parent 
species, with little or no intermixture of the two; and 
such instances do very probably occur. 
