EVOLUTION
661
EVOLUTION
In order first of all to obtain a just estimate of the
influence of selection, it must be pointed out that not
everything that is attributed to selection has origi-
nated through selection. The origin of many pure
breeds (e. g., of pigeons) is unknown, and cannot
therefore without further investigation be ascribed to
selection. Furthermore, many cultivated forms have
arisen through crosses and segregation of characters,
but not through merely strengthening individual
characters. If we restrict our examination only to
well attested facts, we find, first, that nothing new is
brought about by selection; secondly that the maxi-
mum amount in quantitative modification is obtained
in a few generations (mostly in three to five) and that
this amount can only be maintained through constant
selection. In ease selection is stopped, a regression
will follow proportional to the length of time required
for the progress. In short, as far as facts teach us,
new species do not arise by selection. But if qualita-
tive changes were produced by some other cause, se-
lection would probably be a potent principle in order
to explain %vhy some peculiarities survive and others
disappear. The question is: Whether changes in the
environment may furnish such a cause. There can be
no doubt that the environment does influence organ-
isms and mould them in many ways. As proof of this
we need only draw attention to the different forms of
Alpine and valley plants, to the formation of the
leaves of plants according to the himiidity, shadiness,
or sunniness of the habitat, to the influence of light
and temperature on the formation of pigment and
colouring of the surface, to the strange and consider-
able differences produced, for instance, in knot-
weeds by merely changing the environment, and so
forth. But as far as actual experiments show, the
changes of characteristics and niceties of adaptation
go to and fro, as it were, without transgressing definite
ranges of variation. Moreover, it is not at all clear
how discontinuity of species could have arisen " by a
continuoiLS environment, whether acting directly, as
Lamarck would have it, or as a selective agent, as
Darwin would have it" (Bateson), unless one takes
into account the accidental destruction and isolation
of intermediate forms.
In spite of these conclusions it has been assimied that individual differences might lead to the formation of new species under the continuous influence of nat- ural selection. Wasmann's well-known Dinarda- forms may serve as an example. The four forms of the rove-beetle, Dinanla, namely D. Mdrkeli, D. den- tntti, D. Hagensi and D. pygmaa, bear a certain rela- tion with regard to size to the four forms of ants, For- mica rufii, songuinea, exsecUi, fusco-rufibnrhix, and to their nests, in which they live as tolerated guests. D. Mdrkeli, which is 5 mm. long, dwells with F. ruja, which is comparatively large and builds spacious hill- nests. D. dcnUita, which is 4 mm. long, lives with F. ganguinen, which is comparatively large, but builds small earth-nests. D. Hngensi, which is 3-4 mm. long, lives with /•'. exsecta, which is smaller than F.
sanguinea, but builds a fairly roomy hill-nest. D.
pygmwa, which is 3 mm. long, lives with F. fusco-rufi-
barbis, which is relatively small and builds small earth-
nests. Moreover, the three first-named ants are two-
coloured (red and black), and so are the corresponding
LHnarda. The last-
named ant, how-
ever, is of a more
uniform dark col-
our, as is also
the corresponding
Dinarda. Now
comparative zoo-
geography c o n -
tains some indica-
tions according to
which the similar-
ity of colour and
proportion of size
must be at tributed
to actual adapta-
tion. For (1) there
are regions in Cen-
tral Europe in
which only F. san-
guinea with D. den-
lata, and F. rufa
with D. Mdrkeli
are found, whereas
F. exseeta and F. rufibarbis do not harbour any Din-
arda-forms at all. Secondly, there are districts in
which the four forms of Dinarda are living with their
four hosts and yet hardly ever showing transitional
forms. Thirdly, in other parts there are more or less
continuous intermediate forms, D. dentuta-Hagensi
living with F. exsecta, and D. Hagensi-pygmcea living
with F. fusco-rufibarbis. The nearer a Dinarda ap-
proaches the form of D. pygmaa, the more frequently
it is found with F. fusco-rufibarbis. To all this must
be added, that the adaptation in general appears to
have kept pace with the historical freeing of Central
Europe from ice, though numerous exceptions must
be explained by local circumstances, especially by iso-
lation. Considering these facts, we are inclined to be-
lieve that D. pygmaa especially presents an example of
real adaptation in fieri, though this adaptation cannot
be called a progressive one, since the more recent forms,
Hagensi and pyg-
mcea, are only
smaller in size and
of a more uniform
colour. But at the
same time it seems
to us that the
adaptation of the
Dinarda cannot be
considered as an
example to illiLs-
trate specific evo-
lution, because, as
wehaveshown else-
where, there are
many instances in
nature — we men-
tion ordy the races
and other sub-divi-
sionsof the human
species — that like-
wise present differ-
ent degrees of
adaptation far
more pronounced
than that found in the Dinarda, but which are not,
and cannot on that account be, quoted as examples
of the formation of new specific characters.
(2) Single Variations are presumably of far greater importance for the solution of the evolution problem
(Enotiiera C!ig
