192 Roger Verity 
On the other hand the statement made by Turner, according to th 
general belief, that /yl/us is nothing but the hot dry season forn 
of pamphilus and that the latter must necessarily precede it ii 
the spring, is not correct either. True /yl/us is perfectly distinc 
at all seasons, although the features of the | generation are mucl 
less striking at first sigkt than those of the Il. Thus, neither @ 
these views fits facts exactly and I think the truth must b 
sought for in a third phenomenon, the one I have described a 
„exergism“ or „exclusivism“ in the Entom! Record, 1925, p. 10 
In dealing with the Zygaenae | have pointed out that it is impo 
ssible to limit our knowledge of relationships to specific and raci: 
ones. There exists at least one other kind, in which two group 
possess different hereditary features, but are not sterile to on 
another, so that when they meet they interbreed and they onl 
keep distinct because their constitutions are suitable to differer 
surroundings and usually keep them apart from each other. 
will thus be necessary to work out relationship more accuratel 
than has hitherto been done and establish in each case of group 
differing from each other the sort of distinctness they exhibi 
It is, however a mistake to attempt to judge the degree of die 
tinctness from the fact that the features are more or less strikin; 
as has been done too often in the past. Even one of th 
most thourough and clever Entomologist’s has sent me photograph 
of „genitalia“ and asked me to give my opinion as to whethk 
they were specifically distinct or not. My answer is that any kin 
of morphological difference can be suggestive of specific distinctio 
but none can be conclusive as to its existencee. To my mind 
is only on sterility between two groups one can base specit 
distinction, independently of all visible features. Practically or 
is, of course, obliged to make use of the latter to distinguish t 
individuals of the two groups, but one can only come to a defin 
tive conclusion either by experimental breeding of more than 01 
generation, to exclude the grades of fertility capable of producir 
hybrids during as many as three or four successive ones, ori 
inferences drawn from the following observations: When two group 
distinguishable by some feature, live together in some regions Ar 
no transitional individuals are met with, we can conclude they ai 
in reality specifically distinct. When two such groups inhab 
different areas and replace each other entirely, never producing ea 
other’s features, even as extreme individual variations, but th« 
obviously interbreed where they meet along the boundary of the 
areas as shown by transitional individuals found in that zone onl 
we must conclude we have before us a case of exergism, SU 
