igo Annals of the Transvaal Museum 
that go to prove how vast a period is covered by the existence of birds 
as a class, and in contemplation of the problems which this involves one 
cannot avoid the conclusion that it is a want of sagacity in distinguishing 
between the age of the groups that many workers have been led astray. 
The diet of the species is the main factor in the evolution of species, 
especially the diet at the breeding season, since it is not only essential 
to their existence, but around it revolve other factors, such as the adapta- 
tions to procure it, distribution according to its prevalence, the action of 
physical environment in accord with the habits and distribution of the 
species, and the effect of the food itself upon their disposition may pro- 
foundly affect their development and attainment to secondary characters 
which become very important. The permanence of the diet must pro- 
foundly conduce to the establishment of specific characters, and conversely 
where the diet is uncertain there do we find less definite limits to the groups. 
Examination of this factor in detail will I am convinced produce more 
enlightenment than any other; but the effect of physical environment, 
apart from diet, cannot be ignored. Distribution is the key to the solution 
of many problems which have upset the proper arrangement of the minor 
groups; but I do not consider that strict isolation is so essential to the 
separation of species as some recent writers would have us believe, though 
naturally where isolation becomes effective, the result of permanent diver- 
gence comes about, provided there are other factors which will cause 
divergence. I firmly believe that the preponderance of certain factors 
tending to produce certain characters under one condition of environment 
will, in the course of time, result in those characters becoming dominant 
and the species becoming isolated. Where two different conditions of 
environment are adjacent, the facility for the individuals occupying them 
to interbreed is greater than when a species is distributed over a wide area 
in which there is a gradual transition from one condition of environment 
to another which differs in slight degree ; consequently it is to be expected 
that when a division of the individuals inhabiting two adjacent conditions 
of environment takes place, the line of demarcation is likely to become more 
pronounced, and in fact, if these species are interbred we are less likely to 
get fertile progeny than if we were to interbreed geographical species that 
had come about by isolation alone. This must, I think, constitute a strong 
argument in favour of genus splitting ; but on the other hand geographical 
genera also become established in the course of time and assessment of their 
status then only becomes possible by the criterion of the differences to be 
observed in allied genera which occur side by side. This will, of course, raise 
the argument that all species by this arrangement become at once genera ; 
but that this is not so is shown by the fertility of the hybrid progeny of 
pheasants which have been brought together from distant localities; in 
their respective localities they are clearly defined species, not subspecies 
as some taxonomers would have us take them to be, subspecies in its true 
meaning being a species in transition and without definite limits. I believe 
that less offence will be given by recognising the criterion of the frequent 
occurrence of allied species side by side during the breeding season for 
genera and less frequent occurrence, or during the breeding season of one 
only, of allied species side by side for subgenera, as we have not yet 
pursued this line of reasoning sufficiently far to establish the principle. 
