Vernon L. (Vernon Lyman) Kellogg.

Darwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming online

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Online LibraryVernon L. (Vernon Lyman) KelloggDarwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming → online text (page 23 of 38)
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readily recognisable result of creating a great number of
small variations within species limits.

"The results, manifest to any student of the group, of
these two opposing influences are to render difficult the divi-
sion of the order into distinct genera on account of the gen-
eral similarity of structure, and to make difficult the defini-
tion of species on account of the many slight variations
among the individuals from different bird individuals."*

The study of geographic distribution and its influences on

species-forming has not been limited, of course, to living

Geographic organisms alone. In fact, the geologic study

isolation studied Q f distribution and migration of both animals

among past

animals. and plants has given us some of our most

important facts touching the problem of the influence of
isolation on species-transformation. In an interesting paper


on the "geological study of the migration of marine inverte-
brates," Smith 10 has pointed out the general principles and
conditions upon which the interpretation of the geographic
distribution of the marine invertebrates of the earlier geo-
logic ages must be based.

In closing this consideration of the status of geographic

isolation as a factor in species-forming, I should not omit to

Isolation not call attention to the fact, which should be obvi-

an all-sufficient QUS enoue -h to any reader, that isolation in itself
agent of species- J .

forming. cannot be the basic and all-sufficient cause for

the production of specific differentiation, any more than any
selective factor can. The prerequisite in both cases is the
occurrence of variation. What are the variations, and how
are they produced : these are the fundamental questions in
species-forming. Isolation is a tremendously favouring con-
dition but not a primary cause of species-forming. It tends
to help along, to hurry up species disintegration, not to-
initiate it. It is a biological catalytic agent.

In this present connection the pertinent question is, is the
influence of geographical isolation in the cumulation of
variation or intra-specific differentiation due to its com-
pelling in-and-in breeding and hence the fostering and cumu-
lation of fortuitous Darwinian variations occurring in the
comparatively few individuals of the isolated group, or is
there a spur to variation, or even actual production of it along
determinate lines, in the new environmental conditions com-
mon to all the isolated group but inevitably different from
the conditions to which the parent type is exposed ? Is there
a gradual accumulation of differences between the split-off
group and the parent group due to environmental influence
plus in-and-in breeding? If we could reply yes to this
question, we should have a sufficient explanation of
how the isolated group splits rapidly away in many
small, and in a few large, characteristics from the parent


From the foregoing it is obvious that geographical isola-
tion is a proved, effective, and widely applied species-form-
ing factor. So much cannot be said for biologic
isolation) and sexual isolation. 11 The actual existence of
such isolation is not proved by any mass of evi-
dence based on observation, although the theory is by no
means pure speculation; nor are the results of such isola-
tion clearly indicated, although a certain amount of observa-
tion and experimental evidence can be adduced to show
them. The different phases of isolation, not geographic,
called by different names, as biologic, physiolpgic, sexual,
and morphologic isolation, all have reference to some sort
of segregation of individuals of the same species into groups
inside of each of which mating takes place, and among
which little or no cross-breeding occurs, because of varying
habits, or unusual sexual aversion or attraction, or physio-
logical or morphological variation affecting mating. For
example, to take first a sort of combination of geographic
and biologic isolation, Plate 12 points out that there are
twelve species of albatrosses in the southern hemisphere, of
which nine or ten belong to the Australian zoo-geographical
realm and intermingle throughout most of their range. At
breeding time, however, these different species become
segregated in restricted and separate localities so that mating
is always accomplished among different individuals of the
same species, although hybridisation could doubtless obtain
successfully among these closely related albatross forms.
Thus the species characters are kept pure ; the species dis-

An example of pure biological isolation and one within
a single species (which is the sort of isolation we are more
An example of interested in) might be produced in the follow-
how biologic -yy e know O f numerous species of

isolation might s f

work. butterflies which appear in different seasons of

the year in different colour-pattern. This is not a colour


change in individuals but results from an earlier or later
hatching of eggs laid in the autumn or summer before.
These eggs may, indeed, be all of one batch or lot, laid by a
single female. Some of these eggs hatch in the spring ; the
butterflies that come from these spring larvae are of one col-
our-pattern ; some of the eggs, however, delay hatching until
summer; from these larvae come butterflies of another
colour-pattern ; some of the eggs even go over until fall
before hatching ; these latest butterfly individuals may be of
a third colour-pattern. The colour-pattern here must have
some fixed relation to the time or season of hatching of the
eggs ; it is not a result of isolation. But the condition well
illustrates the actual existence of a biological isolation within
a species : the spring butterflies must mate among them-
selves, the summer individuals among themselves, and the
fall butterflies among themselves. Within the one species
are three biologically isolated groups of individuals re-
strained from inter-breeding. Suppose the individuals of
a bird species show among themselves a tendency to vary
in their breeding time ; some are ready to breed early, others
delay mating. Roughly segregated into two groups, early
breeders and late, the individuals of the two groups would
obviously tend to breed each inside its own group. Hut-
ton " actually records the occurrence of two varieties of the
shore-bird, (Estrelata neglecta, in the Kermadec Islands,
which live together but breed at different times. A pelagic
crustacean living near the shore increases rapidly in num-
bers ; some individuals find themselves able to live on the
shore in pools between tide-lines. The pool dwellers breed
together : the pelagic individuals breed together ; a biologic
isolation in truth an isolation partly topographic might
soon come to exist. Any variation in habits of life among
individuals living in the same locality, which tends to deter-
mine that breeding shall be roughly restricted within cer-
tain groups produces biologic isolation ; such variation might


be a variance in sexual maturity, a change in breeding time
for that or any other reason, a tendency on the part of cer-
tain individuals to live more or less concealed in holes, udder
stones, etc., changes in food-habits, as the gradual going
over of some individuals of a plant-feeding insect species
frorp the old food-plant to a new one, or the tendency within
an omnivorous species for groups to restrict themselves to
certain specific foods : all such variations might lead to pos-
sible biological isolation.

By sexual isolation authors usually refer to the influ-
ence of some variation tending to make difficult or impossi-
Sexual td e wholly free and miscellaneous mating or

isolation. breeding inside of a species. This variation

may be of purely physiological character or may be a struc-
tural one : that is, the hindrance to mating may be one of
instinctive feeling, a "race-feeling" depending on an antipa-
thy to odour, to age, to appearance, etc., or may be a slight
modification of the copulatory organs making such mating
difficult, or even a modification of the egg or the spermato-
zoids making fertilisation difficult. It is a well-known fact
that numerous varieties of domesticated animal species
rarely breed together, although quite able to, and provided
with full opportunity. On the other hand, animals of differ-
ent species which in Nature rarely or never breed together
may, if kept long in confinement, as in zoological gardens,
mate 14 and produce young. In each case there seems to be
question of a "race- feeling" ; in the first case a sexual
aversion keeping apart individuals qf the same species, in
the second the breaking down of race-feeling that in Nature
has sufficed to prevent hybridising. This might be termed
physiological isolation, or, indeed, physiological selection, as
it has been called, and given much credit for
species-forming b Y Romanes 1B and others.
Romanes and Hutton believe that a progressive
infertility results in this way (and also by the way referred


to in the next paragraph) which can soon result in complete
infertility, hence specific distinctness on the part of the
mutually infertile groups.

Mutual infertility due to morphological variations has
been called "mechanical selection" by Karl Jordan," and
may rest on slight variations in germ-cells or
Mechanical copulatory structures. Such morphological
variations of the reproductive organs have been
believed to be shown for butterflies and spiders, while the
delicate tropismic responses of the active spermatozoids to
the attracting chemical substances in the egg-cells of echino-
derms, coelenterates, molluscs, and fishes have been thought
to be conditions in which a slight chemical or physical varia-
tion might have a large influence in preventing or insuring
fertilisation. Indeed, when one examines comparatively the
curiously various complex accessory reproductive organs
(claspers, gonapophyses, intromittent organs, etc.) of almost
any group of insects, one's first thought is that this variety
must practically effectually exclude all possibility of hybri-
dising. But the interesting detailed comparative studies
by Snodgrass 1T of the accessory genitalia in various families
of Diptera make this confidence less certain. In the large
family Tipulidse, for example, he finds great complexity
and remarkable variety (among the different closely allied
species) in this complexity in the genitalia of the males, but
almost no variation at all in the corresponding (complement-
ary) parts of the females. "Throughout the entire family
the females present one type of structure, of which there is
but little modification, and certainly none to correspond with
the great variety of specific differences found in the genitalia
of the males." Now while the great variety of the copula-
tory parts would be extremely significant if shared by both
sexes so that only one kind of key could fit one kind of lock,
we have the inexplicable condition actually existing of the
keys being extremely various and complex, but the locks all


being so similar and simple that any bent nail is able to pick

Seebohm 18 criticises Romanes's theory of physiological
selection, which should better be called physiological
Seebohm's isolation, as demanding an almost impossible
""slXiose- coincidence of conditions to make it work.
lection. Formulated as nearly as it can be in a single

sentence, Romanes has defined physiological selection as
follows : "Wherever, among all the possible variations of
the highly variable reproductive system there arises toward
any parent form any degree of sterility which does not
extend to the varietal form, there a new species must neces-
sarily take its origin." Seebohm points out that this is
exactly a condition that can rarely, if ever, occur, for to
bring it about we must presuppose :

"ist. The special variation of the reproductive organs
must occur in two individuals, otherwise the possible an-
cestor of the new species would leave no descendants.

"2d. It must occur at the same time in both.

"3d. It must occur at the same place.

"4th. The two individuals must be of opposite sexes.

"5th. They must each of them possess some other varia-
tion, or their progeny would not differ from that of the rest
of the species.

"6th. The variation must be the same in both or appear
simultaneously in the majority of their children, otherwise
it would be swamped by interbreeding within the physio-
logically isolated family."

Romanes's theory has also been strongly criticised by

Wallace 1 ' and by Karl Jordan. 20 Wallace contends that

"no form of infertility or sterility between the

Other criticisms

ofBomanes's individuals of a species can be increased by
theory. natural selection unless correlated with some

useful variation, while all unfertility not so correlated has a
constant tendency to effect its own elimination. But the


opposite property, fertility, is of vital importance to every
species, and gives the offspring of the individuals which
possess it, in consequence of their superior numbers, a
greater chance of survival in the battle of life. It is, there-
fore, indirectly under the control of natural selection, which
acts both for the self-preservation of fertile and the self-
destruction of unfertile stocks except always, as correlated
above, when they become useful and therefore subject to be
increased by natural selection." Jordan maintains that the
outcome of physiological selection can be at best only
dimorphism, not specific distinctness.

Vernon " has formulated a theory which he calls that of
"reproductive divergence," in the following words: "Sup-
Vemon's posing that among the members of any species
ductiv/div^r- " those individuals more alike in respect to any
genoe. different characteristic, such as colour, form, or

size, are slightly more fertile inter se than less similar indi-
viduals, it necessarily follows that in the course of succeed-
ing generations the members of this species will diverge
more and more in respect to the characteristic in question,
whereby ultimately the original species may be split up into
two or more fresh species." As a concrete example, Vernon
supposes that in the Lepidopterous Ithania urolina, a certain
insect found in the Amazon Valley, small individuals were
slightly more fertile with other small individuals than with
large individuals, while these were also more fertile inter se;
"then it would follow that fewer individuals of intermediate
size would be produced, and in course of time the species
would be split up into a small and large variety. These
varieties would continue to diverge as long as the process
of 'reproductive divergence' was acting, till at length they
might become differentiated in the two mutually sterile
species. Supposing on the other hand this variation in
fertility were correlated with slight differences of colour,
then in course of time varieties differing in respect of colour


would be produced; or if it were correlated with both size
and colour, varieties differing in respect of both characters
might be produced. As a matter of fact, this insect does
actually occur as four distinct varieties differing in colour,
form, and size, though whether in consequence of the oper-
ation of reproductive divergence it is of course impossible
to sa,y." But this theory has been strongly criticised by
Karl Jordan, 22 who believes himself able to show that re-
productive divergence would not work in the way conceived
by Vernon, but actually in such a way as to establish an
intermediate form.

Karl Pearson 2 " has formulated a theory called "reproduc-
tive selection" which he believes to be distinct from both the
Pears n' Romanes and the Vernon theories, and to which
theory of repro- he attributes an importance in evolution "equi-
dnctive selection. potent tQ natural se i ec tion, if indeed it be not

prepotent." The theory is based on correlations which seem
to exist between the variation in some particular organ and
fertility. From studies of variation of height in 4,000 Anglo-
Saxon families and 1,182 Danish families, Pearson finds
that there exists a distinct correlation between fertility and
height in the mothers of daughters in these families. The
effect of this correlation is to render women less variable
and to raise their mean height. The quantities are small,
but are sufficient, if unchecked by natural selection, to raise
the mean height of women in forty generations by 3 1-4
inches. "A factor which would alter stature by about three
inches in 1,000 years is clearly capable of producing very
considerable results in the long periods during which evolu-
tion may be supposed to have been at work."

Of large importance in any consideration of the relations

Gullet's im- of isolation to species-forming are the observa-

portantobBerva- tions and conclusions of Gulick. Derived origi-

tions and conclu-

eiow. nally from an exhaustive study of the variation

and life-conditions of certain land shells (Achatinellidae)


in the Hawaiian Islands, he formulated, in 1872, an im-
portant principle concerning the species-differentiating ef-
fects of indiscriminate isolation. As Romanes 2t well points
out, isolation may not only admit of degrees, that is,
may be either total or partial, and, if partial, may occur
in numberless grades of efficiency, but it may be either dis-
criminate or indiscriminate. If it be discriminate, the isola-
tion has reference to the resemblance of the separated indi-
viduals to one another ; if it be indiscriminate, it has no such
reference. For example, if a shepherd divides a flock of
sheep without regard to their characters, he is isolating one
section from the other indiscriminately; but if he places all
the white sheep in one field and all the black sheep in another
field, he is isolating one section from the other discriminately.
Or, if geological subsidence divides a species into two parts,
the isolation will be indiscriminate ; but if the separation be
due to one of the sections developing, for example, a change
of instinct determining migration to another area, or occu-
pation of a different habitat on the same area, then the isola-
tion will be discriminate, so far as the resemblance of instinct
is concerned. Discriminate isolation has been called by
Gulick segregate breeding, and indiscriminate isolation sepa-
rate breeding.

Now the effectiveness of discriminate isolation or segre-
gate breeding, however effected, to produce species-differ-
entiation is of course obvious. In fact, as Romanes points
out, it is only when assisted by some form of discriminate
isolation which determines the exclusive breeding of like
with like, that heredity can make in favour of change of
type or lead to what we understand by organic evolution.
But what about indiscriminate isolation ? Does it not seem,
at first sight at least, that this kind of isolation must count
for nothing in the process of evolution ? Is it not apparently
self-evident that the farmer who separated his stock into two
or more parts indiscriminately, would not effect any more


change in his stock than if he had left them all to breed
together ?

"Well," says Romanes, "although at first sight this seems
self-evident, it is in fact untrue. For, unless the individuals
which are indiscriminately isolated happen to be a very large
number, sooner or later their progeny will come to differ
from that of the parent type, or unisolated portion of the
previous stock. And, of course, as soon as this change of
type begins, the isolation ceases to be indiscriminate: the
previous apogamy [indiscriminate isolation] has been con-
verted into homogamy [discriminate isolation], with the
usual result of causing a divergence of type. The reason
why progeny of an indiscriminately isolated section of an
originally uniform stock e. g., of a species will eventually
deviate from the original type is, to quote Mr. Gulick, 25 as
follows : "No two portions of a species possess exactly the
same average character, and, therefore, the initial differ-
ences are for ever reacting on the environment and on each
other in such a way as to ensure increasing divergence a?
long as the individuals of the two groups are kept from

Gulick was led to his recognition of the principle in ques-
tion, not by any deductive reasoning front general principles,
Guiick's stud ^ llt ky l" s own particular and detailed observa-
ies of Hawaiian tions of the land mollusca of the Sandwich
land-snails. Islands. Here there is an immense number
of varieties belonging to several genera ; but every variety
is restricted, not merely to the same island, but actually
to the same valley. Moreover, on tracing this fauna
from valley to valley, it is apparent that a slight varia-
tion in the occupants of valley 2 as compared with those
of the adjacent valley i, becomes more pronounced in
the next valley 3, still more so in 4, etc., etc. Thus it
was possible, as Mr. Gulick says, roughly to estimate the
amount of divergence between the occupants of any


two given valleys by measuring the number of miles be-
tween them.

On the basis of his detailed observations, Gulick" has
proposed the following three general propositions as to the
relations of isolation to species-forming:

"i. A species exposed to different conditions in the differ-
ent parts of the area over which it is distributed, is not
represented by divergent forms when free inter-breeding
exists between the inhabitants of the different districts. In
other words, diversity of natural selection without separation
does not produce divergent evolution.

"2. We find many cases in which areas, corresponding in
the character of the environment, but separated from each
other by important barriers, are the homes of divergent
forms of the same or allied species.

"3. In cases where the separation has been long continued,
and the external conditions are the most diverse in points
that involve diversity of adaptation, there we find the most
decided divergences in the organic forms. That is, where
separation and divergent selection have long acted, the re-
sults are found to be the greatest.

"The first and second of these propositions will probably
be disputed by few, if by any. The proof of the second is
found wherever a set of closely allied organisms is so dis-
tributed over territory that each species and variety occu-
pies its own narrow district, within which it is shut by bar-
riers that restrain its distribution, while each species of the
environing types is distributed over the whole territory.
The distribution of terrestrial molluscs on the Sandwich
Islands presents a great body of facts of this kind."

Finally in a recent exhaustive discussion of the subject 2T
of the relation of isolation to evolution Gulick declares that
"the whole process of bionomic evolution, whether progres-
sive or retrogressive, whether increasingly ramified and
divergent, or increasingly convergent through amalgama-


tion, is a process by which the limitations of segregate breed-
ing are either set up and established or cast down and


1 Seebohm, Henry, "Physiological Selection," p. 12, 1886.

2 Romanes, G. J., "Darwin and After Darwin," Vol. III. p. i, 1897.

3 Wagner, Moritz, "Die Entstehung der Arten durch Raumliche
References to Sonderung," 1889. This book is made up of the

discussions of collected papers of Wagner, printed originally in the
isolation. t i me f rom !868-i886, mostly in the journal Kosmos.

* Wagner, Moritz, "Uber den Einfluss der geographischen Isolie-
rung und Kolonienbildung auf die morphologischen Veranderungen
der Organismen," July, 1870.

6 Wagner, Moritz, "Uber die Entstehung der Arten durch Abson-
derung," Kosmos, 1880.

6 Haacke in his "Grundriss der Entwicklungsmechanik," 1897,
gives, on pp. 335-336, an excellent summary statement of Wagner's
position, as follows :

"Wenn wir eine Tierart bis an die Grenze ihres Verbreitungs-
gebietes verfolgen und diese Grenze iiberschreiten, so stossen wir
Haacke's sum- gewohnlich sehr bald, und oft schon, ehe wir die
maryofWag- Verbreitungsgrenze der betreffenden Art erreicht

Online LibraryVernon L. (Vernon Lyman) KelloggDarwinism to-day; a discussion of present-day scientific criticism of the Darwinian selection theories, together with a brief account of the principal other proposed auxilary and alternative theories of species-forming → online text (page 23 of 38)