76 SECTIONAL ADDRESSES. 
a sub-specific name. Similarly a genus is a species, or a number of related 
species, whose distinctive morphological characters entitle it, or them, to 
generic ravk. 
There are, of course, many species so distinct from all others and so 
uniform throughout their range that everyone is agreed about them ; but 
frequently the limits and contents of a species, as of a genus, are a matter of 
opinion. No systematist has, or should have, any rule as to the amount of 
difference required for the recognition of a species or a sub-species ; he 
is guided by convenience. In practice it often happens that geographical 
forms, representing each other in different areas, are given only sub- 
specific rank, even when they are well defined, and that closely related 
forms, not easily distinguished, are given specific rank when they inhabit 
the same area but keep apart. 
I have seen a species defined as a stable complex of genes—or words 
to that effect—and Bateson, without exactly defining a species, has 
insisted that those systematists who distinguish between good and bad 
species are right, and that the distinction between the two is not simply 
a question of degree or a matter of opinion. There is some truth in this ; 
in the absence of exact knowledge seasonal or sexual differences have been 
regarded as specific, and hybrids, as well as varieties that differ from the 
normal in some well-marked character, have been given specific names : 
these are certainly bad species. There is truth also in Bateson’s contention 
that species are qualitatively different from varieties, if we restrict this 
word to the kind of varieties he has specially studied and do not use it for 
communities that differ from each other in morphological characters. 
According to Bateson the principal qualities of species are morpho- 
logical discontinuity amd interspecific sterility ; but to the implication 
that these have been suddenly acquired I would reply that in nature there 
is every gradation from communities that are morphologically indistin- 
guishable to others that are so different that everyone is agreed that they 
are well-marked species; and it is not surprising that when morpho- 
logical differentiation has proceeded to this extent it should generally, 
but not always, be accompanied by mutual infertility. That morpho- 
logical discontinuity in a continuous environment which appears to Bate- 
son to support the theory of the discontinuous origin of specific characters 
is seen to be the final term of a habitudinal discontinuity that began with 
the formation of communities that were at first morphologically identical. 
Bateson’s argument that the Natural Selection Theory, or any theory of 
gradual transformation, demands that the ancestral form from which 
two species have diverged should persist as an intermediate is seen 
to be quite fallacious if we get a firm grip of the idea of the division of a 
species into communities, followed by the evolution of each community 
as a separate entity. 
A great deal of work has been done, especially on our more important 
food-fishes, in making biometrical analyses and investigating the life- 
histories of the different communities. The pioneer research was that 
of Heincke on the herring ; he showed that in the North Sea there were 
several communities, each with its own slight morphological peculiarities, 
its own area, and its own time and place for breeding. Heincke grouped 
these communities into two main classes—herrings of the open sea that 
spawned in summer or autumn in rather deep water of high salinity, 
