1957] Ghouri and McFarland — Reproductive Isolation 35 
Discussion 
These crossing experiments have shown that the Pakis- 
tani and Canadian strains of A. domesticus are repro- 
ductively isolated, and therefore are actually separate 
species, as defined biologically (Mayr et al., 1953). 
In his crossing experiments with the races of A. as- 
similis, Fulton (1952) found that while insemination 
always took place, eggs laid by the females invariably 
failed to hatch. He concluded that hybrids were not pro- 
duced either because the eggs were not fertilized or be- 
cause the zygotes were not viable. With the two strains 
of A. domesticus , however, reproductive isolation consists 
in a failure of insemination to take place. It is possible 
that the strains of A. domesticus are not isolated at the 
genetic level or at the level of fertilization that separates 
the races of A. assimilis; however, attempts to achieve 
artificial insemination or fertilization have not been suc- 
cessful, due to technical difficulties. 
Although there can be little doubt that geographic isola- 
tion has led to the differences between the Pakistani and 
Canadian strains, it would be of interest to discover the 
mechanism which is responsible for their isolation when 
brought together. Behavioural differences, in the broad 
sense, are obviously involved, and these may have to do 
with actions during courtship or perhaps with the odour 
of the male. In A. assimilis , Fulton (1952) has found 
that two “races” may occupy the same habitat, and the 
possibility that a more intensive study of both species 
groups will show that geographic isolation is not essential 
in speciation makes a study of the mechanism of isolation 
very much worthwhile. 
The Pakistani and Canadian “strains” of A. domesticus 
are two distinct species, but, as has been pointed out by 
Gurney (1951) and Hubbell (1956) for A. assimilis , no 
practical purpose will be served, at the present stage of our 
knowledge of these crickets, by creating a new specific 
name. It will be desirable, however, to maintain a distinc- 
tion between the species by giving each a geographic 
designation. 
