49 
male genital organs of closely allied species is well-known. That 
these same organs, within the limits of a well-marked single species, 
often offer considerable variation, is also well-known. That species 
with very distinct looking male genitalia, such as those presented by 
Zggaena Jilipenclulae and Z. trifolii, will pair and produce hybrid 
progeny is a well-ascertained fact. One is uncertain, therefore, how 
much structural difference is necessary to prevent successful pairing 
between, and the production of fertile eggs by, two allied species. So 
little actual experiment in this direction has been performed, that one 
is inclined to reject the statements laid down as veritable axioms, 
that one repeatedly finds in the works of even our best naturalists, 
relating to this point. 
STERILITY NOT A NECESSITY OF SPECIFIC DISTINCTION. 
No one can read Darwin’s remarks on “hybridity,” in the Origin 
of Species , without recognising that he was not at all clear how far 
fertility between allied species was general or the reverse. He was, 
however, evidently quite clear that the ability of two forms to cross 
and to produce fertile progeny, did not render them any the less two 
quite distinct species. Yet he assumed that between first crosses there 
was a tendency to sterility, and that in the intercrossing of the hybrids 
there was a still greater tendency in this direction. In spite of this, 
cases are cited by him in which hybrid plants were as fertile as the 
parent species ; he also cites the well-known case of Phasicinus colchicus 
and P. torquatus, also the case of the Indian humped ox, being perfectly 
fertile with the common ox, in each instance the hybrids also being 
fertile. When one considers the difficulties of breeding animals arti¬ 
ficially, the ill effects of in-breeding, the individual idiosyncrasies of 
each animal, the thousand and one difficulties that have to be sur¬ 
mounted, in order only to attain a fair amount of success •when 
breeding the same species, it appears evident that we require much 
more detailed information before any very sweeping generalisations 
may be formulated. 
INCIPIENT SPECIES AND PARTIAL STERILITY. 
Even Wallace lays it down as a law that, when two incipient 
species are in process of formation, one condition of their differentiation 
as distinct species necessitates “ some amount of infertility when 
crossed with the parent form, or with each other.” Now, it appears 
to me, once the power is granted for certain species to be able to 
hybridise freely, somewhat illogical to insist on this as a general 
principle, and, I must confess that, although I can see the advantage 
to the incipient forms should such a condition arise, yet, I do not at all 
follow the necessity for it. 
Wallace supports his view by asserting that the danger of a 
species, placed under new and adverse conditions, so that it cannot 
adapt itself to them with sufficient rapidity, is much increased if 
crossing with the parent form is not checked and afterwards com¬ 
pletely prevented except as a very occasional occurrence. He looks 
upon the means of preventing inter-crossing as being three in 
number: (1) Infertility. (2) The presence of “recognition marks” 
or external distinctions leading to the preferential mating of similar 
forms. (3) Physical isolation. He believes that the latter is of 
little importance, because the majority of new species must arise in 
the midst of the population of existing species. He thinks, further, 
