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of those distinctive forms that have endured for a long time, 
and to recognize these, and these only, as species, then natural 
selection would be responsible for them all. 
Obviously, our opportunities, our judgment, and our conven- 
ience all combine to make us adopt a middle course. A very 
considerable proportion of new species are described from single 
plants. It is far from unheard-of for two species to be de- 
scribed from a single plant. In the groups that vary widely, of 
which I presume that the genus Crataegus is one, it is probably 
not unusual for more than one species to be described from 
the progeny of a single plant. Up to this time, such a practice 
has not been adopted, except in cases where the parentage is 
unknown. If the same freedom of species-recognition and de- 
scription were practiced with various cultivated plants, for ex- 
ample, tobacco, where the parentage is known but the offspring 
vary beyond the bounds that would be recognized as specific 
among wild plants of unknown parentage, there would be almost 
no limit to the number of species ; but systematic botanists 
have so far mercifully abstained from doing this. 
The responsibility of natural selection for the species recog- 
nized at any moment in a given place depends then very largely 
upon what we recognize as a species. The possible origin of 
species by the summation or selection of variations, whether 
slight or great, is another question to which the answer is fixed 
chiefly by our choice of definitions. In so far as the species 
originates by a single variation — which may always be true, if 
we define species in that way — natural selection is never respon- 
sible for its immediate appearance. Even if we go as far as 
the veriest determinate-variationist might, and assume that 
species N has been derived from the species A, through B, C, D, 
E, etc., all steps being in the same direction, and the most of 
the steps being individually short enough to escape our notice, 
still each of these intermediate forms by the definition just sug- 
gested is itself a species, and N, in its turn, originates by one 
variation from the different parent species M. 
That the species which occupy the world to-day have originated 
by the selective accumulation of relatively small differences, it 
is not worth the mutationist’s while to deny. None of them will 
waste his time looking for an Angiosperm as the mutant of a 
Flagellate, nor will any mutationist be disposed to deny that 
between these extremes there has been a large number of inter- 
mediate steps. If we agree with Doctor Willis that the only 
difference between little steps and big steps is one of degree, 
