iw) 
(7) 
renders dominicanus a variety of niavius and not a distinct 
species. But it is clear that they do more than this; they 
make it almost certain that the two forms freely interbreed, 
and constitute but a single syngamic community. 
This is one of the remarkably clear examples. In many 
cases we know the transition, but the extremes are not sorted 
out in different parts of the total area of distribution. Never- 
theless if complete enough the transition of forms on the 
same area always raises the strong presumption that we are 
dealing with a syngamic community. 
Probably the most remarkable series of transitional varieties 
ever depicted is that shown in the eleven quarto plates of 
the last part of Monsieur Charles Oberthiir’s great “ Etudes 
d’Entomologie,” entitled “ Variation des Heliconia thelxiope et 
vesta”” (Rennes, February, 1902). 
The method of diagnosis, at its clearest and simplest, is 
always consistent with, and often strongly suggests, an under- 
lying syngamy. There are, however, numberless examples 
belonging to various categories in which a rigid adherence 
to diagnosis cannot avail. In these cases the systematist 
frankly appeals to syngamy or synepigony as decisive; and 
if he has not direct proof of the existence of either of these, 
indirect evidence is, at any rate provisionally, regarded as 
sufficient. 
I. Dimorphism, Polymorphism :—In an_ ever-increasing 
number of examples an assemblage of individuals is regarded 
as a single species, although split up into two or more widely 
different and sharply separated groups, between which transi- 
tional varieties are excessively rare or even unknown. For 
instance, the extremely abundant, widely distributed butterfly 
Limnas chrysippus includes among other forms one in which 
the black-and-white tip is wanting from the fore-wing, the 
dorippus (=klugii) form. This variety is sharply cut off from 
the type fcrm. Although faint traces of a former white bar 
can be made out in dorippus, I have never seen, among 
thousands of individuals, the material out of which a good 
transitional series between it and chrysippus could be con- 
structed. In this case the evidence of syngamy is strong and 
complete ; for Col. Yerbury has recorded the fact that the 
