15 
any rate, it would not unlikely be found, that the Japanese or the 
North American representative of this group would breed equally freely 
with either of our British species, if given the chance, and might, with 
equal satisfaction, be referred to either or both. 
I am going to take hold of those words “ if given the chance,” and 
point out that the only thoroughly workable lines of specific demar¬ 
cation are to be found in the investigation, not of what might occur 
but does not, but rather of what does occur in a state of nature at the 
present time. In Prof. Poulton’s paper on “ What is a Species ? ” to 
which I have already referred, several passages prove that he has 
endeavoured to restrict the typical meaning of his word “ syngamy ” 
to this normal occurrence of interbreeding, in accordance with his own 
original definition of the term ( Proc. Ent. Soc. Loud., 1903, p. cx): 
“ Free interbreeding under natural conditions” (the italicizing of the 
word “free” is my own) ; and it is perhaps not inconsistently with 
this that, on p. cviii, he speaks of “ intermittent syngamy ” in the case 
where species by their migratory habits get occasional oppoitunities for 
mixed crossing and thereby stifle the tendency to segregate into 
definitely fixed geographical races. But here and there he seems to be 
satisfied with a very partial degree of contact to justify the unqualified 
use of the word syngamy. Thus on pp. xciv-xcv, after pointing out 
that Amauris niacins of West Africa and the dominicanus form of the 
East and South-east Coasts are very constant in these regions and 
were formerly treated as species, he shows that the discovery of a 
locality (on the North-eastern shores of Lake Victoria Nyanza), where 
the two forms overlap and interbreed, constitutes them “ but a single 
syngamic community.” I do not say that this may not he a perfectly 
warrantable conclusion for one who is au fait with the details of this 
particular question, and it represents the position taken up by so high 
an authority as Dr. Karl Jordan. The differences, I admit, may he no 
greater than the racial ones of which our Paliearctic butterflies, for 
instance, afford hundreds of examples, and where no question of 
independent “ species ” need arise. But I wish to point out that here 
we are approaching the really difficult cases of which I gave some 
hypothetical examples just now. The point is that complete isolation 
or segregation spells complete asyngamy, partial isolation or segregation 
partial asyngamy (or “intermittent syngamy”) and only the absence of 
every kind of barrier, geographical or physiological, a complete or 
perfect syngamy. 
Hence we are driven back upon the question of “isolation,” which 
was mainly the text of an able presidential address delivered before 
this Society, by Mr. J. W. Tutt, exactly twelve years ago (Trans. City 
Land. Ent. Soc., 1896-97, pp. 40-59). Mr. Tutt treated asyngamy as 
an effect, rather than as a cause, of species-divergence, but took a 
very wide view of what kind of isolation might give the original 
stimulus to the divergence, or be the effective agent in the promotion 
of asyngamy. He would apparently not exclude the “ physiological 
selection ” of Romanes as a possible factor, but he rather emphasizes 
other considerations, such as phenological or geographical separation, 
or even segregation to particular foodplants, etc. Dr. Jordan, on the 
other hand (loc. cit., supra), considers that all new species have arisen 
by geographical isolation, and whether we accept absolutely the 
xix. 
