ner's theory. haben, auf eine andere, und zwar auf eine mit der
ersteren nachstverwandte Tierart, die aber ein anderes Verbreitungs-
areal inne hat. Gehen wir auch iiber das Gebiet dieser letzteren
Art hinaus, so konnen wir auf eine dritte, vierte und fiinfte Art
stossen, von denen jede den beiden ersten verwandt sein kann und
ein besonderes Verbreitungsgebiet bewohnt. Im allgemeinen kon-
nen wir den Satz aufstellen, dass es keine zwei nachstverwandten
Tierarten giebt, deren Verbreitungsgebiete sich vollkommen decken.'
Vielfach kann der Fall festgestellt werden, dass die Verbreitungsge-
biete zweier nachstverwandter Tierarten sich teilweise decken; aber
eine voilkommene Deckung ist noch in keinem Fall bei zwei oder
mehr nachstverwandten Tierarten festgestellt worden. Es kann
auch vorkommen, dass das Verbreitungsgebiet der einen Art voll-
standig innerhalb desjenigen der andern Art liegt, das also, soweit
der Wohnkreis der ersten Art reicht, ein Zusammenfallen mit dem
Verbreitungsgebiet der zweiten Art stattfindet ; aber in solchen
Fallen dehnt sich eben die Heimat der einen Art iiber die der
zweiten aus, so dass von Kongruenz der beiderseitigen Wohnge-
biete nicht die Rede sein kann. Nachtsverwandte Tierarten sind
ketten- oder, besser gesagt, netzformig iiber die Erde verbreitet.
Wie die Maschen eines Netzes reihen sich die Wohngebiete der
Arten einer Gattung aneinander, und wenn auch, wie schon hervor-
gehoben, mancherlei teilweise Deckungen vorkommen, so hat sich
noch in alien Fallen, wo man die Grenzen der Verbreitungsgebiete
nachstverwandter Arten festgestellt hat, die Thatsache ergeben, dass
keine vollkommene Deckung stattfindet. Aus dieser Thatsache
konnen wir den Schluss ziehen, dass in einem und demselben
Gebiete, soweit wenigstens alle Individuen unter denselben Ver-
haltnissen leben, aus einer Art nicht zwei oder mehr neue Arten
werden konnen. Wagner meinte zuerst, dass hierbei die Moglich-
keit einer allseitigen Kreuzung, wie sie nach ihm innerhalb eines
und desselben Wohnkreises einer Art moglich sein soil, eine grosse
Rolle spielt. Er hat iibrigens seine Ansichten im Laufe der Zeit
geandert und es ist deshalb notwendig, auf die Entwickelungs-
geschichte seiner Ideen etwas naher einzugehen. Urspriinglich
suchte Wagner seine Theorie mit der Darwin'schen zu vereinigen.
Nach der letzteren entsteht eine neue Art dadurch aus einer
vorhandenen, dass die Leben sbeeinflussungen andere werden, und
dass nunmehr diejenigen Individuen seitens der ziichtendeh Natur
ausgewahlt werden, die den neuen Lebensbeeinflussungen am
besten entsprechen. Wagner nahm nun an, dass dies zunachst nur
einzelne Individuen sein konnen, und dass nicht bloss sie, sondern.
noch eine grosse Anzahl anderer leben bleiben, so dass nicht allein
die Moglichkeit, sondern auch die hohe Wahrscheinlichkeit gegeben.
ist, dass die den neuen Lebensbeeinflussungen am besten ent-
sprechenden, von den iibrigen Individuen der betreffenden Organis-
menart abweichenden Vertreter der letzteren sich mit denjenigen
geschlechtlich mischen, die nicht in zweckentsprechender Weise
abgeandert sind, wodurch die neuen Errungenschaften wieder
verloren gehen sollten. Wagner suchte also den Nachweis zu
fuhren, dass die Darwinische Selektionstheorie nicht geeignet sei,
eine Ziichtung neuer Tier- und Pflanzenarten ohne eine Hiilfslehre,
die er in seiner 'Migrationstheorie' gefunden zu haben glaubte,
nachzuweisen. Er meinte, dass die vorteilhaft abgeanderten
Individuen. wenn nicht in alien, so doch in manchen Fallen auswan-
dern wiirden in eine Gegend, wo die Art, der sie angehoren, nicht
vertreten, wo also die Moglichkeit einer Kreuzung mit unabgean-
derten Individuen ausgeschlossen ist. Spater hat Wagner seine
Migrationstheorie durch die der Separation oder der raumlichen
Sonderung ersetzt, indem er zugleich die Verquickung seiner
Anschauungen mit denen des Darwinismus zuriicknahm. Nach
Wagners Separationstheorie bilden sich neue Arten dadurch, dass
auf die eine oder andere Weise etliche Individuen einer Art in ein
OTHER THEORIES OF SPECIES-FORMING. 255
Gebiet gelangen, das vorher nicht von dieser Art bewohnt war.
Die Entstehung neuer Arten erklart sich dann nach Wagner
dadurch, dass, da die Individuen einer Art ja alle mehr oder
minder voneinander abweichen, die wenigen Griinder der neuerr
Art ihre Besonderheiten bewahren und nicht durch Kreuzung mit
anderen Individuen wieder einbiissen wiirden. Die Anpassung
lasst Wagner aber im Sinne Lamarcks zustande kommen, und
nebei} Einrichtungen, die den Organismen von Nutzen sind,
erkennt er andere an, die lediglich der Ausdruck eigentiimlicher
Struktur sind. Die klimatischen Verhaltnisse sind nach Wagner
von sehr untergeordneter, die Verhinderung der Kreuzung ist
von ausschlaggebender Bedeutung. Wagner huldigte ferner der
Anschauung, dass die weisse Farbe der Polar- und die gelbe der
Wiistentiere dadurch zustande gekommen ist, dass entsprechend
gefarbte Individuen von Arten, die andere Gegenden bewohnten,
nach den Polarlandern und den Wusten auswanderten."
7 Jordan, D. S., "The Origin of Species through Isolation,""
Science, N. S., Vol. XXII, pp. 545-562, 1905.
8 An excellent example of the careful study of the relation of a
group of recognised varieties, or sub-species of a species, to the
Grinnell's study c 'i mat * c differences o f their various geographic
of geographic ranges, is presented by Jos. Grinnell in "The Origin
differences in the and Distribution of the Chestnut-backed Chickadee,""
chickadee. Auk> VoL H> pp 364-382, 1904. I quote from this
paper the following :
"The chestnut-backed chickadee (Pants rufescens) is a boreal
species of peculiarly limited distribution. It is almost exclusively
confined to the humid Pacific Coast region of North America,
within which it is the most abundant, and in many places the
only member of the genus Parus present. We find it characteris-
tically at home within the densest coniferous forests, or along their
edges, where there is much shade and an even temperature.
"The range of the chestnut-backed chickadee is nearly two-
thousand miles long, north and south, extending from a little north
of Sitka, Alaska, to some forty miles below Monterey, California.
But its width is very narrow, only within the confines of Oregon
and Washington exceeding one hundred miles, and elsewhere
usually much less, save for one or two isolated interior colonies
to be mentioned later.
"The influences determining this queer-shaped distribution area
may be safely assumed to be atmospheric humidity, with associated
floral conditions. For this habitat coincides quite accurately with
the narrow coastal belt of excessive cloudy weather and rainfall.
"The specific character distinguishing Parus rufescens from all
256 DARWINISM TO-DAY.
other American chickadees is the colour of the back, which is an
intense rusty-brown, approaching chestnut. It is of common note
that the most evident effects of similar climatic conditions on other
animals is a corresponding intensification of browns, especially
dorsally. We may, therefore, consider the chestnut-backed chicka-
dee, as indicated by its chief specific character, to be a product
exclusively of the peculiar isohumic area to which we find it
"Parus rufescens, from Sitka to Monterey, has a chestnut-coloured
back. And from Sitka to Point Arena, * between which we find
the extremest humidity, another conspicuous character is uniform,
the colour of the sides, which are also deep rusty brown. But from
Point Arena south to San Francisco Bay (Marin District), these
lateral-brown areas suddenly weaken to pale-rusty; while from
San Francisco south past Monterey (Santa Cruz District), adult
birds have the sides pure smoke-gray without a trace of sty.
"The species thus presents geographic variation within itself, and
three distinguishable forms have been named, respectively, the chest-
nut-sided chickadee (Parus rufescens rufescens), the Marin chick-
adee (Parus rufescens neglectus), and the Santa Cruz chickadee
(Parus rufescens barlowi). But all three sub-species are unmis-
takably the chestnut-backed chickadee (Parus rufescens). . . .
"As has already been asserted, Parus rufescens doubtless arose
as a geographical race of Parus pre-liudsonicus [the hypothetical
common ancestor of the present species, Parus hudsonicus, occupy-
ing the interior of Alaska and British Columbia east to Labrador
and Nova Scotia, and Parus rufescens}. It is now called a 'species'
because intermediates have dropped out; in other words, the
divarication is now wholly complete and there are two separate
twigs. The area of intermediate faunal conditions between the
humid coast belt and the arid interior region of British Columbia
and Alaska is very narrow, consisting, in places personally
traversed by me, of but a few miles over a mountain ridge. This
very narrowness of the area of faunal mergence probably accounts
for the lack of intermediates at the present day between hudsonicus
"In the case of Parus rufescens and Parus hudsonicus, there
seems to be now a narrow hiatus between the two. At least I can
find no record of the two species having been found in the same
locality. The narrowness of the region of intermediate faunal
conditions may therefore be considered as the reason why we do
not find connecting links between hudsonicus and rufescens at the
present time. For the amount of difference between these two
chickadees does not strike me as any greater than, for instance,
OTHER THEORIES OF SPECIES-FORMING. 257
between Melospisa cinerea montana and Melospisa cinerea rufina,
between which there is continuous distribution and free interoscula-
tion. But we cannot expect any two species of birds or other
animals to present the same degrees of differentiation in the same
length of time or under the same conditions, much less under
different conditions. For in no two animals is the physical organ-
isation, in all respects, exactly the same.
"In a given aggregation of individuals constituting a new colony,
a certain amount of time is necessary for the set of environmental
factors to become operative in bringing about new inheritable
characters to a degree perceptible to us. Then the inherited effects
of invasion and cross-breeding from season to season from the
adjacent parent centre of differentiation will be evidenced less and
less, as time elapses, as the distance from this centre increases.
The offsoring of successively further removed unions will, of course,
inherit tV, n a less and less degree the distinctive characters of the
ancestral stock on one side and more and more of the incipient
ones on the other.
"If, now, the distance is great enough to permit of the time
required for adaptive manifestations to become innate, then we
would find new characters making their appearance distally nearest
the new centre of differentiation. If the distance were too short
we would not find new characters showing themselves because
they would be constantly crowded down by the influx of the old.
The time factor may, therefore, be reduced by the intervention of
an impassable barrier. As an instance, we find three (and there
are probably two other) insular forms of the song sparrow within
a limited distance among the Santa Barbara Islands, while through
the same distance on the adjacent mainland there is but one. Or,
in the case of continuous distribution, the time element may be
comparatively lessened by the great distance between the range
limits, and it may be still further decreased as these limits lie in
faunal areas of more emphatically different nature. The horned
larks, as well as song sparrows, furnish us several good examples
of the latter two rules.
"It is isolation, either by barriers or by sufficient distance to
more than counterbalance inheritance from the opposite type, that
seems to me to be the absolutely essential condition for the differ-
entiation of two species, at least in birds.
"A strong argument in support of this conviction is that we
never find two 'sub-species' breeding in the same faunal area, and
no two closely similar species, except as can be plainly accounted
for by the invasion of one of them from a separate centre of
differentiation in an adjacent faunal area. An appropriate instance
258 DARWINISM TO-DAY.
in illustration of the latter is the occurrence together, in the Siski-
you Mountains of northern California, of the brown Parus rufescens
of the wet coastal fauna and the gray Parus gambeli of the arid
Sierran fauna. (See Anderson and Grinnell, Proc. Ac. Nat. Sc.,
Phila., 1903, p. 13.) The Siskiyou Mountains occupy a line of
mergence between the two faunae, and the two respectively repre-
sentative chickadees have evidently extended their ranges toward
each other until now over this one small area they occupy com-
mon ground. Several parallel cases could be cited ; their signifi-
cance seems obvious.
"We come now to consider the origin of the races of Parus
rufescens. In a species of recent arrival into a new region (by
invasion from a neighbouring faunal area), as it adapts itself better
and better to its new surroundings, granted the absence of closely-
related or sharply-competing forms, its numbers will rapidly
increase. This means that there will be increased competition within
the species itself, on account of limited food supply. The alterna-
tive results are either starvation for less vigorous individuals
during recurring seasons of unusual food scarcity, or dissemination
over a large area. In a way the first might be considered as bene-
ficial in the long run, as doubtless leading to the elimination of
the weaker; such a process evidently does take place to a greater
or less degree all the time, and is important for the betterment of
the race. But as a matter of observation Nature first resorts to
all sorts of devices to ensure the spreading of individuals over all
inhabitable regions; in other words, the extremest intra-competi-
tion does not ensue until after further dissemination is impossible.
In birds we find a trait evidently developed on purpose to bring
about scattering of individuals. This is the autumnal 'mad im-
pulse' which occurs just after the complete annual moult, when
both birds-of-the-year and adults are in the best physical condition,
and just before the stress of winter food shortage. Even in the
most sedentary of birds, in which no other trace of a migratory
instinct is discernible, this fall season of unrest is plainly in evi-
dence. I may suggest, not unreasonably, that autumnal migration
may have had its origin in such a trait as this, the return move-
ment in the spring becoming a necessary sequence. (See Loomis,
Proc. Cal. Acad. Sc., 3d series, Zoology, II, Dec., 1900, 352.) It
is a matter of abundant observation that autumn is the season when
we find the most unlooked-for stragglers far out of their normal
range and when sober, stay-at-home birds, like Pipilo crissalis and
the chickadees, wander far from the native haunts where they so
closely confine themselves the rest of the year. It is also the expe-
rience of collectors that the greatest number of these stragglers
OTHER THEORIES OF SPECIES-FORMING. 259
are birds-of-the-year, which thus, obeying the 'mad impulse,' are
led away from their birthplace into new country, where they may
take up their permanent abode, and be less likely to compete with
their parents or others of their kind. Then, too, cross-breeding
of distinctly related individuals is more likely. The records of
the Santa Cruz chickadee outside of its regular breeding range, are
all of August to October dates (Hay wards, Gilroy, San Jose, etc.).
"Thtls, as above indicated, by the occupancy of new territory
the number of individuals which can be supported will corre-
spondingly grow. Hence a vigorous colony will spread out along
lines of least resistance, being hindered by slight faunal changes,
but completely checked only by topographic or abrupt climatic
barriers. Parus hudsonicus and its near relative Parus rufescens
are boreal species, the former inhabiting the Hudsonian Zone and
the latter a certain portion of the Canadian. It seems reasonable
to suppose that rufescens differentiated in the northern part of the
humid coast belt, which has been called the Sitkan District. This
is a faunal subdivision of the Canadian Zone, and its northern
part approximates more closely Hudsonian conditions than south-
erly. Granting that the early centre of differentiation and distri-
bution of Parus pre-hudsonicus rufescens was in the northern part
of the Sitkan District, then the route of emigration would be con-
fined to the narrow southward extension of that faunal area. The
habitat of Parus rufescens thus gradually acquired the long north
and south linear appearance, as shown at this day. But when the
pioneer invaders at the south reached the vicinity of Point Arena,
they met with somewhat changed temperature and consequent
floral conditions, but not so abrupt as to constitute a permanent
barrier. Doubtless the progress of invasion was retarded until
adaptive modifications evolved, which correlatively allowed of
further invasion, until the abrupt limits of the Santa Cruz Dis-
trict were reached.
"San Francisco Bay and the Golden Gate seem to now form a
pretty effectual barrier between neglectus on the north and barlowi
on the south. At least, among the large number of skins examined
by me with this point in view, I can find none from one side that
can be confidently determined as being identical with the race on
the other. Neither chickadee has been found east of the bay, nor
anywhere nearly so far from the coast belt, except for one record
of a specimen taken in the fall at Haywards. This has been reex-
amined and proved to be barlowi, as was to be expected from its
contiguity. However, the Golden Gate is so narrow that an occa-
sional crossing may take place. This was more probable formerly,
when the redwood timber grew up to the Gate on both sides.
2 6o DARWINISM TO-DAY.
Heermann, in 1853, recorded the species from 'San Francisco.' But
now, I think, the bird is unknown for several miles on either side
of the Gate. Doubtless this barrier accounts in part for the
origin of the distinct form barloivi within so short a distance. . . .
"As has become a generally accepted idea, the young plumages of
birds, if different at all from those of the adults, present a gener-
alised type of coloration; or, to express it in another way, the
young more nearly resemble recent ancestral conditions. The
familiar examples of the spotted, thrush-like plumage of the young
robin and the streaked, sparrow-like plumage of young towhees
and j uncos are cases in point. Accepting this phylogenetic signifi-
cance of ontogeny, we find the chickadees giving some interesting
"Although the adult of barlowi has the sides pure smoke-gray,
the Juvenal plumage possesses pale-rusty sides. This points
towards a rusty-sided ancestor like neglectus. This also agrees
perfectly with the distributional evidence of origin. The adult of
neglectus has pale-rusty sides ; the young also has rusty sides, but
somewhat darker than in the corresponding age of barlowi, and
moreover is more nearly like the Juvenal plumage of rufescens.
But the sides in adult rufescens are deep brown, almost chestnut,
while the young has much paler, merely dark-rusty sides. And
what is most significant is that the young of rufescens and hud-
sonicus are much nearer alike than are the adults, the former hav-
ing only very slightly darker rusty on the flanks. The young of
hudsonicus in respect to intensity of browns almost exactly equal
the adults of the same species, showing that the present coloration
is of very long standing, and offering further evidence that hudson-
icus is nearest the common stock form of all the chickadees under
consideration. Juvenal characters, resembling ancestral conditions,
lag behind the newer acquired adult characters.
"To repeat: The young of barlowi has the sides paler rusty than
neglectus, neglectus slightly paler than rufescens, but rufescens has
the sides slightly more rusty than hudsonicus, a sequence which
accords well with the present theories of origin."
* Kellogg, V. L., "New Mallophaga, I," Contrib. to Biol. from the
Hopkins Seaside Laboratory of Leland Stanford Jr. University,
10 Smith, Jas. P., "Studies for Students: Geological Study of
Migration of Marine Invertebrates," Journal of Geology, Vol. Ill,
pp. 481-495, 1895.
11 Any selective breeding or segregation produced by other means
than the separation of groups of individuals by actual topographic,
or geographic barriers may be called biologic or sexual isolation.
OTHER THEORIES OF SPECIES-FORMING. 261
12 Plate, L., "Uber die Bedeutung des Darwin'schen Selections-
prinzip," p. 193, 1903.
11 Hutton, F. W., "The Place of Isolation in Or-
Further refer- ganic Evolution," Nat. Science, Vol. XI, pp. 240-246,
enoes to discus-
sions of isolation. I8 97-
14 The not uncommon mating, in zoological gardens,
of lions and tigers, with the production of healthy cubs, is a case
16 This principle, strongly advocated by Romanes, seems first to
have been presented by Eimer in connection with his theory of
orthogenetic evolution. At least it enters into the make-up of the
Eimerian theory. See account of Eimer's theory in chapter x of this
16 Jordan, Karl, "Mechanische Selection," 1896; see also Peter-
sen, Wilh., "Entstehung der Arten durch Physiologische Isolirung,"
Biol. Centralbl., Vol. XXII, pp. 468 ff., 1902; also Vol. XXIV, pp.
423-431, 467-473. 1904. Author describes cases of marked differ-
ence in reproductive organs (includes primary and accessory parts)
of closely allied species of Lepidoptera.
17 Snodgrass, R. E., "The Terminal Abdominal Segments of
Female Tipulidae," Jour. N. Y. Ent. Soc., Vol. XI, pp. 177-183, 1903;
"The Hypopygium of the Tipulidae," Trans. Amer. Ent. Soc., Vol.
XXX, pp. 179-235, 1904; "The Hypopygium of the Dolichopodidae,"
Proc. Cal. Acad. Sci., Ser. 3, Zool., Vol. Ill, pp. 273-285, 1904.
18 Seebohm, H., "Physiological Selection," 1886.
19 Wallace, A. R., "Darwinism," p. 180, 1891.
20 Jordan, Karl, "Novitates Zoologies," pp. 426 ff., 1896.
21 Vernon, H. N., "Reproductive Divergence: An Additional
Factor in Evolution," Natural Science, Vol. XI, pp. 181-189, 1897.
22 Jordan, Karl, "Reproductive Divergence : A Factor in Evolu-
tion?" Natural Science, Vol. II, pp. 317-320, 1897.
23 Pearson, Karl, "Reproductive Selection," Natural Science, Vol.
VIII, pp. 321-325, 1896.
24 Romanes, G. J., "Isolation in Organic Evolution," Monist, Vol.
VIII, pp. 19-38, 1897-
25 Gulick, J. T., "Divergent Evolution through Cumulative Segre-
gation," Jour. Linn. Soc., Zool, Vol. XX, pp. 189-274, 1888.
28 Gulick, J. T., Jour. Linn. Soc., Zool, Vol. XX, pp. 202-211,
27 Gulick, J. T., "Evolution, Racial and Habitudinal," Pub. No.
25, Carnegie Institution of Washington, 1905. In this large paper
are to be found references to all of the author's important papers.
Some of these papers are reprinted (some completely, some in
part) in this monograph.
OTHER THEORIES OF SPECIES-FORMING AND
DESCENT (CONTINUED) : THEORIES ALTER-
NATIVE TO SELECTION.
WE come now to the brief consideration of three general
theories, or groups of theories, which are offered more as
alternative or substitutionary theories for natural selection
than as auxiliary or supporting theories. These groups of
theories are the Lamarckian one, based on the inheritance
of characters acquired individually (ontogenetically) during
the lifetime of the organism due to the effects of use and
disuse and functional stimuli ; the general conception of
orthogenesis variously provided for by Nageli, Eimer,
Jaeckel (metakinesis), and others, and finally the theory of
heterogenesis, suggested by von Kolliker, definitely formu-
lated by Korschinsky, and most recently, and importantly,
developed by de Vries. Few biologists would hold any of
these theories to be exclusively alternative with natural
selection; de Vries himself would restrict natural selection
but little in its large and effective control or determination
of the general course of descent. But all of these theories
offer distinctly substitutional methods of species-forming,
and one of them includes certainly the most favoured expla-
nation, next to selection, of adaptation, while the authors or
later up-holders of some of them actually deny any con-