for Systematic Biology. 
273 
seemed to be no need to alter our methods. But is it true — that 
we still only see apparently fixed species ? Is it not a fact that, 
as our collections grow and as our work becomes more exact and 
our comparisons closer, variability is being revealed to us all along 
the line until it is folly to continue to work as if we could ignore 
it. Only continue your collection — say of Lepidoptera — and you 
will find, as the collection of my friend Mr Elwes has amply 
demonstrated, that the 4 species ’ melt away into apparently 
endless ‘ varieties.’ In the stony Corals, the variation is so great 
that any attempt at genetic grouping into species can only be the 
purest guesswork. No two workers would do it alike. Our 
types have become ridiculous. They are not the types of any- 
thing in nature : their special value is purely historical ; they 
were the forms accidentally first described. In the great majority 
of animal groups, then, variability is being revealed in some cases 
but slowly, but in others the moment any competent person under- 
takes to describe a really large collection. On all sides, indeed, 
we are hearing demands for some more elastic system of work than 
that supplied us by the Linnean binominal species name. The com- 
plexities of the organic world due to the procession of life through 
time are clearly too great to be investigated by so clumsy and 
indefinable a unit. 
It is I think, then, obvious that we must cease to use a purely 
ideal genetic group such as the Linnean ‘species’ as a unit for 
work. We must have one which more nearly fulfils the require- 
ments already laid down, it must at least be an ascertainable 
fact of Nature. One such unit we have and as far as I can see 
only one, viz. that supplied us by the form, each form being 
an aggregate of structural characters regarded in the abstract. 
The different forms assumed by living matter are the units with 
which we must work. 
Now this conclusion that the form is the only possible unit 
for accurate scientific work is not only what my practical work 
with the stony Corals resulted in but it is what we might have 
arrived at theoretically. For only the form can be the unit in 
evolutionary classification. Organic evolution means nothing 
more than the gradual modification of relatively simpler forms 
in various directions resulting in the production of relatively 
more complex and specialised forms. The forms as such, that is, 
in the abstract, are the only important things for the evolutionist 
and morphologist. It is not easy to keep this clear and to 
convince others of it. The fact that the individual concrete 
forms of life possess the power of almost exactly reproducing 
themselves, sometimes through many generations and over great 
areas, somewhat dazzles us. These great armies of living beings, 
reproducing themselves so far as we can see exactly, have natur- 
