EVOLUTION AS THE RESULT OF EXTERNAL AND INTERNAL FACTORS
We come now to a consideration of other theories that have been advanced
to account for the evolution of new forms; and in so far as these new
forms are adapted to their environment, the theories will bear directly
on the question of the origin of adaptive variations. One school of
transformationists has made the external world and the changes taking
place in it the source of new variations. Another school believes that
the changes arise within the organism itself. We may examine these two
points of view in turn.
The Effect of External Influences
We have already seen that Lamarck held as a part of his doctrine of
transformation that the changes in the external world, the environment,
bring about, directly, changes in the organism, and he believed that all
plants and many of the lower animals have evolved as the result of a
reaction of this sort. This idea did not originate with Lamarck,
however, since before him Buffon had advanced the same hypothesis, and
there cannot be much doubt that Lamarck borrowed from his patron,
Buffon, this part of his theory of evolution.
This idea of the influence of the external world as a factor inducing
changes in the organism has come, however, to be associated especially
with the name of Geoffroy Saint-Hilaire, whose period of activity,
although overlapping, came after that of Lamarck. The central idea of
Geoffroy’s view was that species of animals and plants undergo change as
the environment changes; and it is important to note, in passing, that
he did not suppose that these changes were always for the benefit of the
individual, _i.e._ they were not always adaptive. If they were not, the
forms became extinct. So long as the conditions remain constant, the
species remains constant; and he found an answer in this to Cuvier’s
argument, in respect to the similarity between the animals living at
present in Egypt and those discovered embalmed along with mummies at
least two thousand years old. Geoffroy Saint-Hilaire said, that since
the climatic conditions of Egypt had remained exactly the same during
all these years, the animals of Egypt would also have remained
unchanged.
Geoffroy’s views were largely influenced by his studies in systematic
zoology and by his conception of a unity of plan running through the
entire animal kingdom. His study of embryology and paleontology had led
him to believe that present forms have descended from other organisms
living in the past, and in this connection his discovery of teeth in the
jaws of the embryo of the baleen whale and also his discovery of the
embryonic dental ridges in the upper and in the lower jaws of birds,
were used with effect in supporting the theory of change or evolution.
Lastly, his remarkable work in the study of abnormal forms prepared the
way for his conception of sudden and great changes, which he believed
organisms capable of undergoing. He went so far in fact, in one
instance, as to suppose that it was not impossible that a bird might
have issued fully equipped from the egg of a crocodile. Such an extreme
statement, which seems to us nowadays only laughable, need not prejudice
us against the more moderate parts of his speculation.
His study of the fossil gavials found near Caen led him to believe that
they are quite distinct from living crocodiles. He asked whether these
old forms may not represent a link in the chain that connects, without
interruption, the older inhabitants of the earth with animals living at
the present time. Without positively affirming that this is the case, he
did not hesitate to state that a transformation of this sort seemed
possible to him. He said: “I think that the process of respiration
constitutes an acquirement so important in the ‘disposition’ of the
forms of animals, that it is not at all necessary to suppose that the
surrounding respiratory gases become modified quickly and in large
amount in order that the animal may become slowly modified. The
prolonged action of time would ordinarily suffice, but if combined with
a cataclysm, the result would be so much the better.”
He supposed that in the course of time respiration becomes difficult and
finally impossible as far as certain systems of organs are concerned.
The necessity then arises and creates another arrangement, perfecting or
altering the existing structures. Modifications, fortunate or fatal, are
created which through propagation are continued, and which, if
fortunate, influence all the rest of the organization. But if the
modifications are injurious to the animals in which they have appeared,
the animals cease to exist, and are replaced by others having a
different form, and one suited to the new circumstances.
The comparison between the stages of development of the individual and
the evolution of the species was strongly impressed on the mind of
Geoffroy. He says: “We see, each year, the spectacle of the
transformation in organization from one class into another. A batrachian
is at first a fish under the name of a tadpole, then a reptile
(amphibian) under that of a frog.” “The development, or the result of
the transformation, is brought about by the combined action of light and
of oxygen; and the change in the body of the animal takes place by the
production of new blood-vessels, whose development follows the law of
the balancing of organs, in the sense, that if the circulating fluids
precipitate themselves into new channels there remains less in the old
vessels.” By preventing tadpoles from leaving the water, Geoffroy claims
that it has been shown that they can be prevented from changing into
frogs. The main point that Geoffroy attempts to establish is no doubt
fairly clear, but the way in which he supposes the change to be effected
is not so clear, and his ideas as to the way in which new change may be
perpetuated in the next generation are, from our more modern point of
view, extremely hazy. It is perhaps not altogether fair to judge his
view from the standpoint of the origin of adaptive structures, but
rather as an attempt to explain the causes that have brought about the
evolution of the organic world.
During the remainder of the nineteenth century there accumulated a large
number of facts in relation to the action of the external conditions in
bringing about changes in animals and plants. Much of this evidence is
of importance in dealing with the question of the origin of organic
adaptation.
The first class of facts in this connection is that of geographical
variation in animals and plants. It will be impossible here to do more
than select some of the most important cases. De Varigny, in his book on
“Experimental Evolution,” has brought together a large number of facts
of this kind, and from his account the following illustrations have been
selected. He says: “When the small brown honey-bee from High Burgundy is
transported into Bresse—although not very distant—it soon becomes larger
and assumes a yellow color; this happens even in the second generation.”
It is also pointed out that the roots of the beet, carrot, and radish
are colorless in their wild natural state, but when brought under
cultivation they become red, yellow, etc. Vilmorin has noted that the
red, yellow, and violet colors of carrots appear only some time after
the wild forms have been brought under cultivation. Moquin-Tandon has
seen “gentians which are blue in valleys become white on mountains.”
Other cases also are on record in which the colors of a plant are
dependent on external conditions.
The sizes of plants and animals are also often directly traceable to
certain external conditions; the change is generally connected with the
amount of food obtainable. “Generally speaking,” De Varigny says,
“insular animals are smaller than their continental congeners. In the
Canary Islands the oxen of one of the smallest islands are smaller than
those on the others, although all belong to the same breed, and the
horses are also smaller, and the indigenous inhabitants are in the same
case, although belonging to a tall race. It would seem that in Malta
elephants were very small,—fossil elephants, of course,—and that during
the Roman period the island was noted for a dwarf breed of dogs, which
was named after its birthplace, according to Strabo. In Corsica, also,
horses and oxen are very small, and _Cervus corsicanus_, the indigenous
deer, is quite reduced in dimensions; ... and lastly, the small
dimensions of the Falkland horses—imported from Spain in 1764—are
familiar to all. The dwarf rabbits of Porto Santo described by Darwin
may also be cited as a case in point.”
These facts, interesting as they are, will, no doubt, have to be more
carefully examined before the evidence can have great value, for it is
not clear what factor or factors have produced the decrease in size of
these animals.
The following cases show more clearly the immediate effect of the
environment: “Many animals, when transferred to warm climates, lose
their wool, or their hairy covering is much reduced. In some parts of
the warmer regions of the earth, sheep have no wool, but merely hairs
like those of dogs. Similarly, as Roulin notices, poultry have, in
Columbia, lost their feathers, and while the young are at first covered
with a black and delicate down, they lose it in great part as they grow,
and the adult fowls nearly realize Plato’s realistic description of
man—a biped without feathers. Conversely, many animals when transferred
from warm to cold climates acquire a thicker covering; dogs and horses,
for instance, becoming covered with wool.”
A number of kinds of snails that were supposed to belong to different
species have been found, on further examination, to be only varieties
due to the environment. “Locard has discovered through experiments that
_L. turgida_ and _elophila_ are mere varieties—due to environment—of the
common _Lymnæa stagnalis_.” He says, “These are not new species, but
merely common aspects of a common type, which is capable of modification
and of adaptation according to the nature of the media in which it has
to live.” It has also been shown by Bateson that similar changes occur
in _Cardium edule_, and other lamellibranchs are known to vary according
to the nature of the water in which they live.
In regard to plants, the influence of the environment has long been
known to produce an effect on the form, color, etc., of the individuals.
“The common dandelion (_Taraxacum densleonis_) has in dry soil leaves
which are much more irregular and incised, while they are hardly dentate
in marshy stations, where it is called _Taraxacum palustre_.
“Individuals growing near the seashore differ markedly from those
growing far inland. Similarly, species such as some Ranunculi, which can
live under water as well as in air, exhibit marked differences when
considered in their different stations, as is well known to all. These
differences may be important enough to induce botanists to believe in
the existence of two different species when there is only one.”
An interesting case is that of _Daphnia rectirostris_, a small
crustacean living sometimes in fresh water, at other times in water
containing salt and also in salt lakes. There are two forms,
corresponding to the conditions under which they live, and it is said
that the differences are of a kind that suffice to separate species from
each other. In another crustacean, _Branchipus ferox_, the form differs
in a number of points, according to whether it lives in salt or in fresh
water. Schmankewitsch says that, had he not found all transitional
forms, and observed the transformation in cultures, he would have
regarded the two forms as separate species. The oft-quoted case of
Artemia furnishes a very striking example of the influence of the
environment. _Artemia salina_ lives in water whose concentration varies
between 5 and 12 degrees of saltness. When the amount of salt is
increased to 12 degrees, the animal shows certain characteristics like
those of _Artemia milhausenii_, which may live in water having 24 to 25
degrees of saltness. The form _A. salina_ may be further completely
changed into that of _A. milhausenii_ by increasing the amount of salt
to the latter amount.
Among domesticated animals and plants—a few instances of which have been
already referred to—we find a large number of cases in which a change in
the environment produces definite changes in the organism. Darwin has
made a most valuable collection of facts of this kind in his “Animals
and Plants under Domestication.” He believes that domesticated forms are
much more variable than wild ones, and that this is due, in part, to
their being protected from competition, and to their having been removed
from their natural conditions and even from their native country. “In
conformity with this, all our domesticated productions without exception
vary far more than natural species. The hive-bee, which feeds itself,
and follows in most respects its natural habits of life, is the least
variable of all domesticated animals.... Hardly a single plant can be
named, which has long been cultivated and propagated by seed, that is
not highly variable.” “Bud-variation ... shows us that variability may
be quite independent of seminal reproduction, and likewise of reversion
to long-lost ancestral characters. No one will maintain that the sudden
appearance of a moss-rose on a Provence rose is a return to a former
state, ... nor can the appearance of nectarines on peach trees be
accounted for on the principle of reversion.” It is said that
bud-variations are also much more frequent on cultivated than on wild
plants.
Darwin adds: “These general considerations alone render it probable that
variability of every kind is directly or indirectly caused by changed
conditions of life. Or to put the case under another point of view, if
it were possible to expose all the individuals of a species during many
generations to absolutely uniform conditions of life, there would be no
variability.”
In some cases it has been observed that, in passing from one part of a
continent to another, many or all of the forms of the same group and
even of different groups change in the same way. Allen’s account of the
variations in North American birds and mammals furnishes a number of
striking examples of this kind of change. He finds that, as a rule, the
birds and mammals of North America increase in size from the south
northward. This is true, not only for the individuals of the same
species, but generally the largest species of each genus are in the
north. There are some exceptions, however, in which the increase in size
is in the opposite direction. The explanation of this is that the
largest individuals are almost invariably found in the region where the
group to which the species belongs receives its greatest numerical
development. This Allen interprets as the hypothetical “centre of
distribution of the species,” which is in most cases doubtless also its
original centre of dispersal. If the species has arisen in the north,
then the northern forms are the largest; but if it arose in the south,
the reverse is the case. Thus, most of the species of North America that
live north of Mexico are supposed to have had a northern origin, as
shown by the circumpolar distribution of some of them and by the
relationship of others to Old World species; and in these the largest
individuals of the species of a genus are northern. Conversely, in the
exceptional cases of increase in size toward the south, it can be shown
that the forms have probably had a southern origin.
The Canidæ (wolves and foxes) have their largest representatives, the
world over, in the north. “In North America the family is represented by
six species, the smallest of which (speaking generally) are southern and
the largest northern.” The three species that have the widest ranges
(the gray wolf, the common fox, and the gray fox) show the most marked
differences in size. The skull, for instance, of “the common wolf is
fully one-fifth larger in the northern parts of British America and
Alaska than it is in northern Mexico, where it finds the southern limit
of its habitat. Between the largest northern skull and the largest
southern skull there is a difference of about thirty-five per cent of
the mean size. Specimens from the intermediate region show a gradual
intergradation between the extremes, although many of the examples from
the upper Missouri country are nearly as large as those from the extreme
north.” The common fox is about one-tenth larger, on the average, in
Alaska than it is in New England. The gray fox, whose habitat extends
from Pennsylvania southward to Yucatan, has an average length of skull
of about five inches in the north, and less than four in Central
America—about ten per cent difference.
The Felidæ, or cats, “reach their greatest development as respects both
the number and the size of the species in the intertropical regions.
This family has sent a single typical representative, the panther
(_Felis concolor_), north of Mexico, and this ranges only to about the
northern boundary of the United States. The other North American
representatives of the family are the lynxes, which in some of their
varieties range from Alaska to Mexico.” Although they vary greatly in
different localities in color and in length and texture of pelage, they
do not vary as to the size of their skulls. On the other hand the
panther (and the ocelots) greatly increases in size southward, “or
toward the metropolis of the family.”
Other carnivora that increase in size northward are the badger, the
marten, the fisher, the wolverine, and the ermine, which are all
northern types.
Deer are also larger in the north; in the Virginia deer the annually
deciduous antlers of immense size reach their greatest development in
the north. The northern race of flying squirrels is one-half larger than
the southern, “yet the two extremes are found to pass so gradually one
into the other, that it is hardly possible to define even a southern and
a northern geographical race.” The species ranges from the arctic
regions to Central America.
In birds also similar relations exist, but there is less often an
increase in size northward. In species whose breeding station covers a
wide range of latitude, the northern birds are not only smaller, but
have quite different colors, as is markedly the case in the common
quail, the meadow-lark, the purple grackle, the red-winged blackbird,
the flicker, the towhee bunting, the Carolina dove, and in numerous
other species. The same difference is also quite apparent in the blue
jay, the crow, in most of the woodpeckers, in the titmice, numerous
sparrows, and several warblers and thrushes. The variation often amounts
to from ten to fifteen per cent of the average size of the species.
Allen also states that certain parts of the animal may vary
proportionately more than the general size, there being an apparent
tendency for peripheral parts to enlarge toward the warmer regions,
_i.e._ toward the south. “In mammals which have the external ears
largely developed—as in the wolves, foxes, some of the deer, and
especially the hares—the larger size of this organ in southern as
compared with northern individuals of the same species, is often
strikingly apparent.” It is even more apparent in species inhabiting
open plains. The ears of the gray rabbit of the plains of western
Arizona are twice the size of those of the Eastern states.
In birds the bill especially, but also the claws and tail, is larger in
the south. In passing from New England southward to Florida the bill in
slender-billed forms becomes larger, longer, more attenuated, and more
decurved; while in short-billed forms the southern individuals have
thicker and larger bills, although the birds themselves are smaller.
The remarkable changes and gradations of color in birds in different
parts of North America are very instructive, and the important results
obtained by American ornithologists form an interesting chapter in
zoology. The evidence would convince the most sceptical of the
difficulty of distinguishing between Linnæan species. It is not
surprising to find in this connection a leading ornithologist
exclaiming, “if there really are such things as species.” The
differences here noted are mainly from east to west. We may briefly
review here a few striking cases selected from Coues’s “Key to North
American Birds.”
The flicker, or golden-winged woodpecker (_Colaptes auratus_), has a
wide distribution in eastern North America. It is replaced in western
North America (from the Rocky Mountains to the Pacific) by _C.
mexicanus_. In the intermediate regions, Missouri and the Rocky Mountain
region, the characters of the two are blended in every conceivable
degree in different specimens. “Perhaps it is a hybrid, and perhaps it
is a transitional form, and doubtless there are no such things as
species in Nature.... In the west you will find specimens _auratus_ on
one side of the body, _mexicanus_ on the other.” There is a third form,
_C. chrysoides_, with the wings and tail as in _auratus_, and the head
as in _mexicanus_, that lives in the valley of the Colorado River, Lower
California, and southward.
In regard to the song-sparrow (_Melospiza_), Coues writes: “The type of
the genus is the familiar and beloved song-sparrow, a bird of constant
characters in the east, but in the west is split into numerous
geographical races, some of them looking so different from typical
_fasciata_ that they have been considered as distinct species, and even
placed in other genera. This differentiation affects not only their
color, but the size, relative proportions of parts, and particularly the
shape of the bill; and it is sometimes so great, as in the case of _M.
cinerea_, that less dissimilar looking birds are commonly assigned to
different genera. Nevertheless the gradation is complete, and affected
by imperceptible degrees.... The several degrees of likeness and
unlikeness may be thrown into true relief better by some such
expressions as the following, than by formal antithetical phrases: (1)
The common eastern bird commonly modified in the interior into the
duller colored (2) _fallax_. This in the Pacific watershed, more
decidedly modified by deeper coloration,—broader black streaks in (3)
_hermanni_, with its diminutive local race (4) _samuelis_, and more
ruddy shades in (5) _guttata_ northward, increasing in intensity with
increased size in (6) _rafina_. Then the remarkable (7) _cinerea_,
insulated much further apart than any of the others. A former American
school would probably have made four ‘good species,’ (1) _fasciata_, (2)
_samuelis_, (3) _rafina_, (4) _cinerea_.”
Somewhat similar relations are found in three other genera of finches.
Thus Passerella is “imperfectly differentiated”; Junco is represented by
one eastern species, but in the west the stock splits up into numerous
forms, “all of which intergrade with each other and with the eastern
bird. Almost all late writers have taken a hand at Junco, shuffling them
about in the vain attempt to decide which are ‘species’ and which
‘varieties.’ All are either or both, as we may elect to consider them.”
In the distribution of the genus Pipilo similar relations are found.
There is an eastern form much more distinct from the western forms than
these are from each other.
Finally may be mentioned the curious variations in screech-owls of the
genus Scops. This owl has two strikingly different plumages—a mottled
gray and a reddish brown, which, although very distinct when fully
developed, yet “are entirely independent of age, season, or sex.” There
is an eastern form, _Scops asio_, that extends west to the Rocky
Mountains. There is a northwestern form, _S. kennicotti_, which in its
red phase is quite different from _S. asio_, but in its gray plumage is
very similar. The California form, _S. benderii_, is not known to have a
red phase, and the gray phase is quite different from that of _S. asio_,
but like the last form. The Colorado form, _S. maxwellæ_, has no red
phase, “but on the contrary the whole plumage is very pale, almost as if
bleached, the difference evident in the nestlings even.” The Texas form,
_S. maselli_, has both phases, and is very similar to _S. asio_. The
Florida form is smaller and colored like _S. asio_. The red phase is the
frequent, if not the usual, one. The flammulated form, _S. fiammula_, is
“a very _small species_, with much the general aspect of an ungrown _S.
asio_.” This is the southwestern form, easily distinguished on account
of its small size and color from the other forms.
These examples might be greatly increased, but they will suffice, I
think, to convince one of the difficulty of giving a sharp definition to
“species.” The facts speak strongly in favor of the transmutation
theory, and show us how a species may become separated under different
conditions into a number of new forms, which would be counted as new
different species, if the intermediate forms were exterminated.
In discussing the nature of the changes that bring about variability,
Darwin remarks: “From a remote period to the present day, under climates
and circumstances as different as it is possible to conceive, organic
beings of all kinds, when domesticated or cultivated, have varied. We
see this with the many domestic races of quadrupeds and birds belonging
to different orders, with goldfish and silkworms, with plants of many
kinds, raised in various quarters of the world. In the deserts of
northern Africa the date-palm has yielded thirty-eight varieties; in the
fertile plains of India it is notorious how many varieties of rice and
of a host of other plants exist; in a single Polynesian island,
twenty-four varieties of the breadfruit, the same number of the banana,
and twenty-two varieties of the arum, are cultivated by the natives. The
mulberry tree of India and Europe has yielded many varieties serving as
food for the silkworm; and in China sixty-three varieties of the bamboo
are used for various domestic purposes. These facts, and innumerable
others which could be added, indicate that a change of almost any kind
in the conditions of life suffices to cause variability—different
changes acting on different organisms.”
Darwin thinks that a change in climate alone is not one of the potent
causes of variability, because the native country of a plant, where it
has been longest cultivated, is where it has oftenest given rise to the
greatest number of varieties. He thinks it also doubtful that a change
in food is an important source of variability, since the domestic pigeon
has varied more than any other species of fowl, yet the food has been
always nearly the same. This is also true for cattle and sheep, whose
food is probably much less varied in kind than in the wild species.
Another point of interest is raised by Darwin. He thinks, as do others
also, that the influence of a change in the conditions is cumulative, in
the sense that it may not appear until the species has been subjected to
it for several generations. Darwin states that universal experience
shows that when new plants are first introduced into gardens they do not
vary, but after several generations they will begin to vary to a greater
or less extent. In a few cases, as in that of the dahlia, the zinnia,
the Swan River daisy, and the Scotch rose, it is known that the new
variations only appeared after a time. The following statement by Salter
is then quoted, “Every one knows that the chief difficulty is in
breaking through the original form and color of the species, and every
one will be on the lookout for any natural sport, either from seed or
branch; that being once obtained, however trifling the change may be,
the result depends on himself.” Jonghe is also quoted to the effect that
“there is another principle, namely, that the more a type has entered
into a state of variation, the greater is the tendency to continue doing
so, and the more it has varied from the original type, the more is it
disposed to vary still further.” Darwin also quotes with approval the
opinion of the most celebrated horticulturist of France, Vilmorin, who
maintained that “when any particular variation is desired, the first
step is to get the plant to vary in any manner whatever, and to go on
selecting the most variable individuals, even though they vary in the
wrong direction; for the fixed character of the species being once
broken, the desired variation will sooner or later appear.”
Darwin also cites a few cases where animals have changed quite quickly
when brought under domestication. Turkeys raised from the eggs of wild
species lose their metallic tints, and become spotted with white in the
third generation. Wild ducks lose their true plumage after a few
generations. “The white collar around the neck of the mallard becomes
much broader and more irregular, and white feathers appear in the
duckling’s wings. They increase also in size of body.” In these cases it
appears that several generations were necessary in order to bring about
a marked change in the original type, but the Australian dingoes, bred
in the Zoological Gardens, produced puppies which were in the first
generation marked with white and other colors.
The following cases from De Varigny are also very striking. The dwarf
trees from Japan, for the most part conifers, which may be a hundred
years old and not be more than three feet high, are in part the result
“of mechanical processes which prevent the spreading of the branches,
and in part of a starving process which consists in cutting most roots
and in keeping the plant in poor soil.”
As an example of the sudden appearance of a new variation the following
case is interesting. A variety of begonia is recorded as having appeared
quite suddenly at a number of places at the same time. In another case a
narcissus which had met with adverse circumstances, and had then been
supplied with a chemical manure in some quantity, began to bear double
flowers.
Amongst animals the following cases of the appearance of sudden
variations are pointed out by De Varigny. “In Paraguay, during the last
century (1770), a bull was born without horns, although his ancestry was
well provided with these appendages, and his progeny was also hornless,
although at first he was mated with horned cows. If the horned and the
hornless were met in fossil state, we would certainly wonder at not
finding specimens provided with semi-degenerate horns, and representing
the link between both, and if we were told that the hornless variety may
have arisen suddenly, we should not believe it and we should be wrong.
In South America also, between the sixteenth and eighteenth centuries
the niata breed of oxen sprang into life, and this breed of bulldog oxen
has thriven and become a new race. So in the San Paulo provinces of
Brazil, a new breed of oxen suddenly appeared which was provided with
truly enormous horns, the breed of franqueiros, as they are called. The
mauchamp breed of sheep owes its origin to a single lamb that was born
in 1828 from merino parents, but whose wool, instead of being curly like
that of its parents, remained quite smooth. This sudden variation is
often met with, and in France has been noticed in different herds.”
The ancon race of sheep originated in 1791 from a ram born in
Massachusetts having short crooked legs and a long back. From this one
ram by crossing, at first with common sheep, the ancon race has been
produced. “When crossed with other breeds the offspring, with rare
exception, instead of being intermediate in character, perfectly
resemble either parent; even one of twins has resembled one parent and
the second the other.”
Two especially remarkable cases remain to be described. These are the
Porto Santo rabbit and the japanned peacock. Darwin has given a full
account of both of these cases. “The rabbits which have become feral on
the island of Porto Santo, near Madeira, deserve a fuller account. In
1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board
which had produced young during the voyage, and he turned them all out
on the island. These animals soon increased so rapidly that they became
a nuisance, and actually caused the abandonment of the settlement.
Thirty-seven years subsequently, Cada Mosto describes them as
innumerable; nor is this surprising, as the island was not inhabited by
any beast of prey, or by any terrestrial mammal. We do not know the
character of the mother rabbit; but it was probably the common domestic
kind. The Spanish peninsula, whence Zarco sailed, is known to have
abounded with the common wild species at the most remote historical
period; and as these rabbits were taken on board for food, it is
improbable that they should have been of any peculiar breed. That the
breed was well domesticated is shown by the doe having littered during
the voyage. Mr. Wollaston, at my request, brought two of these feral
rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home
three more specimens in brine and two alive. These seven specimens,
though caught at different periods, closely resemble each other. They
were full-grown, as shown, by the state of their bones. Although the
conditions of life in Porto Santo are evidently highly favorable to
rabbits, as proven by their extraordinarily rapid increase, yet they
differ conspicuously in their small size from the wild English
rabbit.... In color the Porto Santo rabbit differs considerably from the
common rabbit; the upper surface is redder, and is rarely interspersed
with any black or black-tipped hairs. The throat and certain parts of
the under surface, instead of being pure white, are generally gray or
leaden color. But the most remarkable difference is in the ears and
tail. I have examined many fresh English rabbits, and the large
collection of skins in the British Museum from various countries, and
all have the upper surface of the tail and the tips of the ears clothed
with blackish gray fur; and this is given in most works as one of the
specific characters of the rabbit. Now in the seven Porto Santo rabbits
the upper surface of the tail was reddish brown, and the tips of the
ears had no trace of the black edging. But here we meet with a singular
circumstance: in June, 1861, I examined two of these rabbits recently
sent to the Zoological Gardens and their tails and ears were colored as
just described; but when one of their dead bodies was sent to me in
February, 1863, the ears were plainly edged, and the upper surface of
the tail was covered with blackish gray fur, and the whole body was much
less red; so that under the English climate this individual rabbit had
recovered the proper color of its fur in rather less than four years.”
Another striking case of sudden variation is found in the peacock. It is
all the more remarkable because this bird has hardly varied at all under
domestication, and is almost exactly like the wild species living in
India to-day. Darwin states: “There is one strange fact with respect to
the peacock, namely, the occasional appearance in England of the
‘japanned’ or ‘black-shouldered’ kind. This form has lately been named,
on the high authority of Mr. Slater, as a distinct species, viz. _Pavo
nigripennis_, which he believes will hereafter be found wild in some
country, but not in India, where it is certainly unknown. The males of
these japanned birds differ conspicuously from the common peacock in the
color of their secondary wing-feathers, scapulars, wing-coverts, and
thighs, and are, I think, more beautiful; they are rather smaller than
the common sort, and are always beaten by them in their battles, as I
hear from the Hon. A. S. G. Canning. The females are much paler-colored
than those of the common kind. Both sexes, as Mr. Canning informs me,
are white when they leave the egg, and they differ from the young of the
white variety only in having a peculiar pinkish tinge on their wings.
These japanned birds, though appearing suddenly in flocks of the common
kind, propagate their kind quite truly.”
In two cases, in which these birds had appeared quite suddenly in flocks
of the ordinary kind, it is recorded that “though a smaller and weaker
bird, it increased to the extinction of the previously existing breed.”
Here we have certainly a remarkable case of a new species suddenly
appearing and replacing the ordinary form, although the birds are
smaller, and _are beaten in their battles_.
Darwin has given an admirably clear statement of his opinion as to the
_causes of variability_ in the opening paragraph of his chapter dealing
with this topic in his “Animals and Plants.” Some authors, he says,
“look at variability as a necessary contingent on reproduction, and as
much an original law as growth or inheritance. Others have of late
encouraged, perhaps unintentionally, this view by speaking of
inheritance and variability as equal and antagonistic principles. Pallas
maintained, and he has had some followers, that variability depends
exclusively on the crossing of primordially distinct forms. Other
authors attribute variability to an excess of food, and with animals, to
an excess relatively to the amount of exercise taken, or again, to the
effects of a more genial climate. That these causes are all effective is
highly probable. But we must, I think, take a broader view, and conclude
that organic beings, when subjected during several generations to any
change whatever in their condition, tend to vary; the kind of variation
which ensues depending in most cases in a far higher degree on the
nature of the constitution of the being, than on the nature of the
changed conditions.”
Most naturalists will agree, in all probability, with this conclusion of
Darwin’s. The examples cited in the preceding pages have shown that
there are several ways in which the organisms may respond to the
environment. In some cases it appears to affect all the individuals in
the same way; in other cases it appears to cause them to fluctuate in
many directions; and in still other cases, without any recognizable
change in the external conditions, new forms may suddenly appear, often
of a perfectly definite type, that depart widely from the parent form.
For the theory of evolution it is a point of the first importance to
determine which of these modes of variation has supplied the basis for
evolution. Moreover, we are here especially concerned with the question
of how adaptive variations arise. Without attempting to decide for the
present between these different kinds of variability, let us examine
certain cases in which an immediate and adaptive response to the
environment has been described as taking place.
Responsive Changes in the Organism that adapt it to the New Environment
There is some experimental evidence showing that sometimes organisms
respond directly and adaptively to certain changes in the environment.
Few as the facts are, they require very careful consideration in our
present examination. The most striking, perhaps, is the acclimatization
to different temperatures. It has been found that while few active
organisms can withstand a temperature over 45 degrees C., and that for
very many 40 degrees is a fatal point, yet, on the other hand, there are
organisms that live in certain hot springs where the temperature is very
high. Thus, to give a few examples, there are some of the lower plants,
nostocs and protococcus forms, that live in the geysers of California at
a temperature of 93 degrees C., or nearly that of boiling water.
Leptothrix is found in the Carlsbad springs, that have a temperature of
44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in
water between 54 and 68 degrees, and in the hot springs in the
Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland
at 98 degrees.
It is probable from recent observations of Setchel that most of the
temperatures are too high, since he finds that the water at the edge of
hot springs is many degrees lower than that in the middle parts.
The snail, _Physa acuta_, has been found in France living at a
temperature of 35 to 36 degrees; another snail, Paludina, at Abano,
Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54
degrees; Anguillidæ at Ischia at 81 degrees; _Cypris balnearia_, a
crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of
“Pise” at 38 degrees.
Now, there can be little doubt that these forms have had ancestors that
were like the other members of the group, and would have been killed had
they been put at once into water of these high temperatures, therefore
it seems highly probable that these forms have become specially adapted
to live in these warm waters. It is, therefore, interesting to find that
it has been possible to acclimatize animals experimentally to a
temperature much above that which would be fatal to them if subjected
directly to it. Dutrochet (in 1817) found that if the plant, nitella,
was put into water at 27 degrees, the currents in the protoplasm were
stopped, but soon began again. If put now into water at 34 degrees they
again stopped moving, but in a quarter of an hour began once more. If
then put into water at 40 degrees the currents again slowed down, but
began again later.
Dallinger (in 1880) made a most remarkable series of experiments on
flagellate protozoans. He kept them in a warm oven, beginning at first
at a temperature of 16.6 degrees C. “He employed the first four months
in raising the temperature 5.5 degrees. This, however, was not
necessary, since the rise to 21 degrees can be made rapidly, but for
success in higher temperatures it is best to proceed slowly from the
beginning. When the temperature had been raised to 23 degrees, the
organisms began dying, but soon ceased, and after two months the
temperature was raised half a degree more, and eventually to 25.5
degrees. Here the organisms began to succumb again, and it was necessary
repeatedly to lower the temperature slightly, and then to advance it to
25.5 degrees, until, after several weeks, unfavorable appearances
ceased. For eight months the temperature could not be raised from this
_stationary point_ a quarter of a degree without unfavorable
appearances. During several years, proceeding by slow stages, Dallinger
succeeded in raising the organisms up to a temperature of 70 degrees C.,
at which the experiment was ended by an accident.”[27]
Footnote 27:
Quoted from Davenport’s “Experimental Morphology.”
Davenport and Castle carried out a series of experiments on the egg of
the toad, in which they tried to acclimatize the eggs to a temperature
higher than normal. Recently laid eggs were used; one lot kept at a
temperature of 15 degrees C., the other at 24-25 degrees C. Both lots
developed normally. At the end of four weeks the temperature point at
which the tadpoles were killed was determined. Those reared at a
temperature of 15 degrees C. died at 41 degrees C., or below; those
reared at 24-25 degrees C. sustained a temperature 10 degrees higher; no
tadpole dying in this set under 43 degrees C. “This increased capacity
for resistance was not produced by the dying off of the less resistant
individuals, for no death occurred in these experiments during the
gradual elevation of the temperatures in the cultures.” The increased
resistance was due, therefore, to a change in the protoplasm of the
individuals. It was also determined that the acquired resistance was
only very gradually lost (after seventeen days’ sojourn in cooler
water). The explanation of this result may be due, in part, to the
protoplasm containing less water at higher temperatures, for it is known
that while the white of egg (albumen) coagulates at 56 degrees C. in
aqueous solution; with only 18 per cent of water it coagulates between
80 degrees and 90 degrees C.; and with 6 per cent, at 145 degrees C.;
and without water between 100 degrees and 170 degrees C.
It has long been known that organisms in the dry condition resist a much
higher temperature. The damp uredospore is killed at 58.5 degrees to 60
degrees C.; but dry spores withstand 128 degrees C. It is also known
that organisms may become acclimatized to cold through loss of water,
but we lack exact experimental data to show to what extent this can be
carried.
There are also some experiments that go to show that animals may become
attuned to certain amounts of light, but the facts in this connection
will be described in another chapter.
Some important results have been obtained by accustoming organisms to
solutions containing various amounts of salts. A number of cases of this
sort are given by De Varigny. It has been found that littoral marine
animals that live where the water may become diluted by the rain, or by
rivers, survive better when put into fresh water than do animals living
farther from the shore. Thus the oyster, the mussel, and the snail,
Patella, withstand immersion in fresh water better than other animals
that live farther out at sea. The reverse is also true; fresh-water
forms, such as Lymnæa, Physa, Paludina, and others may be slowly
acclimatized to water containing more salt. The forms mentioned above
could be brought by degrees into water containing 4 per cent of salt,
which would have killed the animals if they had been brought suddenly
into it. Similar results have been obtained for amœba.
It has been shown that certain rotifers and tardigrades, and also some
unicellular animals, that live in pools and ponds that are liable to
become dry, withstand desiccation, while other members of the same
groups, living in the sea, do not possess this power of resistance.
Cases of this sort are usually explained as cases of adaptation, but it
has not been shown experimentally that resistance to drying can be
acquired by a process of acclimatization to this condition. The case is
also in some respects different from the preceding, since intermediate
conditions are less likely to be met with, or to be of sufficiently long
duration for the animal to become acclimatized to them. It seems more
probable, in such cases, that these forms have been able to live in such
precarious conditions from the beginning because they could resist the
effects of drying, not that they have slowly acquired this power.
Finally, there must be discussed the question of the acclimatization to
poisons, to which an individual may be rendered partially immune. The
point of special importance in this connection is that the animal may be
said to respond adaptively to a large number of substances, which it has
never met before in its individual history, or to which its ancestors
have never been subjected. It may become slowly adapted to many
different kinds of injurious substances. These cases are amongst the
most important adaptive individual responses with which we are familiar,
and the point cannot be too much emphasized that organisms have this
latent capacity without ever having had an opportunity to acquire it
through experience.
The preceding groups of phenomena, included under the general heading of
individual acclimatization, have one striking thing in common, namely,
that a physiological adaptation is brought about without a corresponding
change in form, although we must suppose that the structure has been
altered in certain respects at least. The form of the individual remains
the same as before, but so far as its powers of resistance are concerned
it is a very different being.
In regard to the perpetuation of the advantages gained by means of this
power of adaptation, it is clear in those cases in which the young are
nourished during their embryonic life by the mother, that, in this way,
the young may be rendered immune to a certain extent, and there are
instances of this sort recorded, especially in the case of some
bacterial diseases. Whether this power can also be transmitted through
the egg, in those instances in which the egg itself is set free and
development takes place outside the body, has not been shown. In any
case, the effect appears not to be a permanent one and will wear off
when the particular poison no longer acts. It is improbable, therefore,
that any permanent contribution to the race could be gained in this way.
Adaptations of this sort, while of the highest importance to the
individual, can have produced little direct effect on the evolution of
new forms, although it may have been often of paramount importance to
the individuals to be able to adapt themselves, or rather to become able
to resist the effect of injurious substances. The important fact in this
connection is the wonderful latent power possessed by all animals. So
many, and of such different kinds, are the substances to which they may
become immune, that it is inconceivable that this property of the
organism could ever have been acquired through experience, no matter how
probable it may be made to appear that this might have occurred in
certain cases of fatal bacterial diseases. And if not, in so many other
cases, why invent a special explanation for the few cases?
We may defer the general discussion of the rôle that external factors
have played in the adaptation of organisms, until we have examined some
of the theories which attribute changes to internal factors. The idea
that something innate in the living substance itself has served as the
basis for evolution has given rise to a number of different hypotheses.
That of the botanist Nägeli is one of the most elaborately worked out
theories of this sort that has been proposed, and may be examined by way
of illustration.
Nägeli’s Perfecting Principle
Nägeli used the term _completing principle_ (“Vervollkommungsprincip”)
to express a tendency toward perfection and specialization.
Short-sighted writers, he says, have pretended to see in the use of this
principle something mystical, but on the contrary it is intended that
the term shall be employed in a purely physical sense. It represents the
law of inertia in the organic realm. Once set in motion, the
developmental process cannot stand still, but must advance in its own
direction. Perfection, or completion, means nothing else than the
advance to complicated structure, “but since persons are likely to
attach more meaning to the word _perfection_ than is intended, it would
perhaps be better to replace it with the less objectionable word
_progression_.”
Nägeli says that Darwin, having in view only the condition of
adaptation, designates that as more complete which gives its possessor
an advantage in the battle for existence. Nägeli claims that this is not
the only criterion that applies to organisms, and it leaves out the most
important part of the phenomenon. There are two kinds of completeness
which we should keep distinctly apart: (1) the completeness of
organization characterized by the complication of the structure and the
most far-reaching specialization of the parts; (2) the completeness of
the adaptation, present at each stage in the organization, which
consists in the most advantageous development of the organism (under
existing conditions) that is possible with a given complication of
structure and a given division of functions.
The first of these conceptions Nägeli always calls “completeness”
(Vollkommenheit), for want of a simpler and better expression; the
second he calls adaptation. By way of illustrating the difference
between the two, the following examples may be given. The unicellular
plants and the moulds are excellently adapted each to its conditions of
life, but they are much less complete in structure than an apple tree,
or a grape vine. The rotifers and the leeches are well adapted to their
station, but in completeness of structure they are much simpler than the
vertebrates.
If we consider only organization and division of labor as the work of
the completing principle, and leave for the moment adaptation out of
account, we may form the following picture of the rise of the organic
world. From the inorganic world there arose the simplest organic being
thinkable, being little more than a drop of substance. If this underwent
any change at all, it would have been necessarily in the direction of
greater complication of structure; and this would constitute the first
step in the upward direction. In this way Nägeli imagines the process
once begun would continue. When the movement has reached a certain
point, it must continue in the same direction. The organic kingdom
consists, therefore, of many treelike branches, which have had a common
starting-point. Not only does he suppose that organisms were once
spontaneously generated, and began their first upward course of
development, but the process has been repeated over and over again, and
each time new series have been started on the upward course. The organic
kingdom is made up, therefore, of all degrees of organization, and all
these have had their origins in the series of past forms that arose and
began their upward course at different times in the past. Those that are
the highest forms at the present time represent the oldest series that
successfully developed; the lowest forms living at the present time are
the last that have appeared on the scene of action.
Organisms, as has been said, are distinguished from one another, not
only in that one is simpler and another more complicated, but also in
that those standing at the same stage of organization are unequally
differentiated in their functions and in their structure, which is
connected primarily with certain external relations which Nägeli calls
adaptations.
Adaptation appears at each stage of the organization, which stage is,
for a given environment, the most advantageous expression of the main
type that was itself produced by internal causes. For this condition of
adaptation, a sufficient cause is demanded, and this is, as Nägeli tries
to show later, the result of the inherited response to the environment.
In many cases this cause will continue to act until complete adaptation
is gained; in other cases, the external conditions give a direction
only, and the organism itself continues the movement to its more perfect
condition.
The difference between the conception of the organic kingdom as the
outcome of mechanical causes on the one hand, or of competition and
extermination on the other hand, can be best brought out, Nägeli thinks,
by the following comparison of the two respective methods of action.
There might have been no competition, and no consequent extermination in
the plant kingdom, if from the beginning the surface of the earth had
continually grown larger in proportion as living things increased in
numbers, and if animals had not appeared to destroy the plants. Under
these conditions each germ could then have found room and food, and have
unfolded itself without hinderance. If now, as is assumed to be the case
on the Darwinian theory, individual variations had been in all
directions, the developmental movement could not have gone beyond its
own beginnings, and the first-formed plants would have remained swinging
now on one side and now on another of the point first reached. The whole
plant kingdom would have remained in its entirety at its first stage of
evolution, that is, it would never have advanced beyond the stage of a
naked drop of plasma with or without a membrane. But, according to the
further Darwinian conception, competition, leading to extermination, is
capable of bringing such a condition to a higher stage of development,
since it is assumed that those individuals which vary in a beneficial
direction would have an advantage over those that have not taken such a
step, or have made a step backward.
If, on the other hand, under the above-mentioned conditions of
unrestricted development, without competition, variations were
determined by “_mechanical principles_,” then, according to Nägeli’s
view, all plant forms that now exist would still have evolved, and would
be found living at the present time, but along with all those that now
exist there would be still other forms in countless numbers. These would
represent those forms which have been suppressed. On Nägeli’s view
competition and suppression do not produce new forms, but only weed out
the intermediate forms. He says without competition the plant kingdom
would be like the Milky Way; in consequence of competition the plant
kingdom is like the firmament studded with bright stars.
The plant kingdom may also be compared to a branched tree, the ends of
whose branches represent living species. This tree has an inordinate
power of growth, and if left to itself it would produce an impenetrable
tangle of interwoven branches. The gardener prevents this crowding by
cutting away some of the parts, and thus gives to the tree distinct
branches and twigs. The tree would be the same without the watchful
trimming of the gardener, but without definite form.
Nägeli states: “From my earlier researches I believe that the external
influences are small in comparison to the internal ones. I shall speak
here only of the influences of climate and of food, which are generally
described as the causes of change, without however any one’s having
really determined whether or not a definite result can be brought about
by these factors. Later I shall speak of a special class of external
influences which, according to my view, bring forth beyond a doubt
adaptive changes.”
The external influence of climate and of food act only as transitory
factors. A rich food supply produces fat, lack of food leads to
leanness, a warm summer makes a plant more aromatic, and its fruit
sweeter; a cold year means less odor and sour fruit. Of two similar
seeds the one sown in rich soil will produce a plant with many branches
and abundance of flowers; the other, planted in sandy soil, will produce
a plant without branches, with few flowers, and with small leaves. The
seeds from these two plants will behave in exactly the same way; they
have inherited none of the differences of their parents. Influences of
this sort, even if extending over many generations, have no permanent
effect. Alpine plants that have lived since the ice age under the same
conditions, and have the characters of true high-mountain plants, lose
these characters completely during the first summer, if transplanted to
the plains. Moreover, it makes no difference whether the seed or the
whole plant itself be transferred. In place of the dwarfed, unbranched
growth, and the reduced number of organs, the plant when transferred to
the plains shoots up in height, branches strongly, and produces numerous
leaves and flowers. The plants retain their new characters as long as
they live in the plain without any other new variation being observed in
them.
Other characteristics also, which arise from different kinds of external
influences due to different localities, such as dampness and shade, a
swampy region, or different geological substrata, last only so long as
the external conditions last.
These transient peculiarities make up the characters of local varieties.
That they have no permanency is intelligible, since they exhibit no new
characters, but the change consists mainly in the over- or
under-development of those peculiarities that are dependent on external
influences. The effect of these influences may be compared to an elastic
rod, which, however much it may be distorted by external circumstances,
returns again to its original form as soon as released.
Besides these temporary changes, due to external influences, there are
many cases known in which the same plant lives under very diverse
conditions and yet remains exactly the same. For example, the species of
_Rhododendron ferragineum_ lives on archæan mountains and especially
where the soil is poor in calcium. Another species, _Rhododendron
hirsutum_ is found especially on soil rich in calcium. The difference in
the two species has been supposed to depend on differences in the soil,
and if so, we would imagine that, if transplanted for a long time, the
one should change in the direction of the other. Yet it is known that
the rusty rhododendron may be found in all sorts of localities, even on
dry, sunny, calcareous rocks of the Apennines and of the Jura, and
despite its residence in these localities, since the glacial epoch, no
change whatever has taken place.
Single varieties of the large and variable genus of _Hieracium_ have
lived since the glacial period in the high regions of the Alps,
Carpathians, and in the far north, and also in the plains of different
geological formations, but these varieties have remained exactly the
same, although on all sides there are transitional forms leading from
these to other varieties.
Some parasitic species also furnish excellent illustrations of the same
principle. Besides the several species of Orobanchia and of the
parasitic moulds, the mistletoe deserves special mention. It lives on
both birch and apple trees and on both presents exactly the same
appearance; and even if it is true that mistletoe growing on conifers
presents certain small deviations in its character, it is still doubtful
whether, if transferred to the birch or apple tree, it would not lose
these differences, thus indicating that they are not permanent.
It is a fact of general observation that, on the one hand, the same
variety occurs in different localities and under different surroundings,
and, on the other hand, that slightly different varieties live together
in the same place and therefore under the same external conditions. It
is evident, then, that food conditions have neither originated the
differences nor kept them up. The rarer cases in which in different
localities different varieties exist show nothing, because competition
and suppression keep certain varieties from developing where it would be
possible otherwise for them to exist.
Nägeli says his conclusion may be tested from another point of view. If
food conditions, as is generally supposed, have a definite, _i.e._ a
permanent, effect on the organism, then all organisms living under the
same conditions should show the same characters. Indeed, it has been
claimed in some instances that this is actually the case. Thus it is
stated that dry localities cause plants to become hairy, and that
absence of hairiness is met with in shady localities. This may apply to
certain species, but in other cases exactly the reverse is true, and
even the same species behaves differently in different regions, as in
_Hieracium_. And so it is with all characteristics which are ascribed to
external influences. As soon as it is supposed a discovery has been made
in this direction, we may rest assured that in other cases the reverse
will be found to hold. We have had, in respect to the influence of the
outer world on organisms, the same experience as with the rules for the
weather,—when we come to examine the facts critically there are found to
be as many exceptions as confirmations of the rule.
If climatic influence has a definite effect, the entire flora of a
special locality ought to have the same peculiarities, but this stands
in contradiction to all the results of experience. The character of the
vegetation is not determined by the environment of the plants but by
their prehistoric origin, and as the result of competition. Nägeli
concludes his discussion with the statement that all of our experience
goes to show that the effects of external influences (climate and food)
appear at once, and their results last only as long as the influences
themselves last, and are then lost, leaving nothing permanent behind.
This is true even when the external influences have lasted for a long
time,—since the glacial epoch, for instance. We find, he claims, nothing
that supports the view that such influences are inherited.
If we next examine the question of changes from _internal causes_,
Nägeli claims that here also observation and research fail to show the
origin of a new species, or even of a new variety from external causes.
In the organic world little change has taken place, he believes, since
the glacial epoch. Many varieties have even remained the same throughout
the whole intervening time; and while it cannot be doubted that new
varieties have also been formed, yet the cause of their origin cannot be
empirically demonstrated. The permanent, hereditary characters, of whose
origin we know something from experience, belong to the individual
changes which have appeared under cultivation in the formation of
domestic races. These are for the most part the result of crossing. So
far as we have any definite information as to the origin of the changes,
they are the result of inner, and never of external, causes. We
recognize that this must be the case, since under the same external
conditions individuals behave differently—in the same flower-bud some
seeds give rise to plants like the parent, others to altered ones. The
strawberry with a single leaflet, instead of three, arose in the last
century in a single individual amongst many other ordinary plants. From
the ten seeds of a pear Van Mons obtained as many different kinds of
pears. The most conclusive proof of the action of inner causes is most
clearly seen when the branches of the same plant differ. In Geneva a
horse-chestnut bore a branch with “filled” flowers, and from this
branch, by means of cuttings, this variation has been carried over all
Europe. In the Botanic Garden at Munich there is a beech with small
divided leaves; but one of its branches produces the common broad
undivided leaves. Many such examples have been recorded which can only
be explained by assuming that a cell, or a group of cells, like those
from which the other branches arose, have become changed in some unknown
way as the result of inner causes. The properties that are permanent and
inherited are contained in the idioplasm, which the parent transmits to
its offspring. A cause that permanently transforms the organism must
also transform the idioplasm. How powerless, in comparison to internal
causes, the external causes are is shown most conclusively in grafting.
The graft, although it receives its nourishment through the stock, which
may be another species, remains itself unchanged.
Nägeli makes the following interesting comparison between the
development of the individual from an egg, and the evolution, or
development, of the phylum. No one will doubt that the egg during the
entire time of its process of transformation is guided by internal
factors. Each successive stage follows with mechanical necessity from
the preceding. If an animal can develop from inner causes from a drop of
plasma, why should not the entire evolutionary process have also been
the outcome of developmental inner causes? He admits that there is a
difference in the two cases in that the plasma that forms the egg has
come from another animal, and contains all the properties of the
individual in a primordial condition. In the other case we must suppose
that the original drop of plasma did not contain at first the primordium
of definite structures, but only the ability to form such. Logically the
difference is unimportant. The main point is that in the primordium of
the germ a special peculiarity of the substance is present which by
forming new substances grows, and changes as it grows, and the one
change of necessity excites the next until finally a highly organized
being is the result.
Nägeli discusses a question in this connection, which, he says, has been
unnecessarily confused in the descent theory. Since we are entirely in
the dark as to how much time has been required for the formation of
phyla, so also are we ignorant as to how long it may have taken for each
step in advance. We may err equally in ascribing too much and too little
time to the process. It is, moreover, not necessary that for every step
the same amount of time should have been required. On the contrary, the
probability is that recognizable changes may at times follow each other
rapidly, and then for a time come to a standstill,—just as in the
development of the individual there are periods of more rapid and others
of less rapid change.
A more difficult problem than that relating to the sort of changes the
external influences bring about in the organism, is the question as to
how they effect the organism, or how they act on it mechanically. This,
as is well known, was answered by Darwin, who regards all organization
as a problem of adaptation: only those chance variations surviving which
are capable of existence, the others being destroyed. On this theory
external influences have only a negative or a passive action, namely, in
setting aside the unadapted individuals. Nägeli, on the other hand,
looks upon some kinds of external conditions as directly giving rise to
the adaptive characters of the organism. This is accomplished, he
supposes, in the following ways: two kinds of influence are recognized;
_the direct action_, which, as in inorganic nature, comes to an end when
the external influences come to an end, as when cold diminishes the
chemical actions in the plant; and _the indirect action_, generally
known as a stimulus, which starts a series of molecular motions,
invisible to us, but which we recognize only in their effects. Very
often the stimulus starts only a reflex action, usually at the place of
application.
A stimulus acting for but a short time produces no lasting effect on the
idioplasm. A person stung by a wasp suffers no permanent effect from the
injury. But if a stimulus acts for a long time, and through a large
number of generations, then it may, even if of small strength, so change
the _idioplasm_, that a tendency or disposition capable of being seen
may be the result. This appears to be the case in regard to the action
of light, which causes certain parts of the plant to turn toward it and
others away from it; also for the action of gravity, which determines
the downward direction of the roots. It may be claimed, perhaps, that
these are the results of direct influence and not of an internal
response, but this is not the case; for some plants act in exactly the
opposite way, and send a stem downward, as in the case of the
cleistogamous flowers of _Cardamine chenopodifolia_; and other plants
turn away from the light. This means that the idioplasm behaves
differently in different plants in response to the same stimulus.
Concerning the more visible effects of adaptation, Nägeli states that in
regard to some of them there can be no question as to how they must have
arisen. Protection against cold, by the formation of a thick coat of
hair, is the direct result of the action of the cold on the skin of the
animal. The different weapons of offence and of defence, horns, spurs,
tusks, etc., have arisen, he maintains, through stimulus to those parts
of the body where these structures arise.
The causes of the other adaptations, especially of those occurring in
plants, are less obvious. Land plants protect themselves from drying by
forming a layer of cork over the surface. The most primitive plants were
water plants, which acclimated themselves little by little to moist, and
then to dry, air. When they first emerged from the water the drying
acted as a stimulus on the surface, and caused it to harden in the same
way as a drop of glue hardens. This hardening in turn acted as a
stimulus, causing a chemical transformation of the surface into a corky
substance. This effect was inherited, and in this way the power to form
cork originated.
Land plants have, in addition to the soft parts, the hard bast and wood
which serves the mechanical purpose of supporting the soft tissues and
protecting them from being injured. The arrangement of the hard parts is
such as to suggest that they are the result of the action of pressures
and tensions on the plant, for the strongest cells are found where there
is most need for them. It is easy to imagine, Nägeli adds, that this
important arrangement of the tissues is the result of external forces
which brought about the result in these parts.
Nägeli accounts for the origin of twining plants as follows. Being
overshadowed by other plants, the stem will grow rapidly in the damp
air. Coming in contact with the stems of other plants, the delicate stem
is stimulated on one side, and grows around the point of contact. This
tendency becomes inherited, and the habit to twine is ultimately
established.
The difference in the two sides of leaves is explained by Nägeli as the
result of the difference in the illumination of the two sides. This
influence of light on the leaf has been inherited. The formation of the
tubular corolla that is seen in many plants visited by insects is
explained as the result of the stimulus produced by the insects in
looking for the pollen. The increase in the length of the proboscis of
the insect is the result of the animal straining to reach the bottom of
the ever elongating tube of the corolla. “The tubular corolla and the
proboscis of the insect appear as though made for each other. Both have
slowly developed to their present condition, the long tube from a short
tube and the long proboscis from a short one.” Thus, by purely
Lamarckian principles, Nägeli attempts to account for many of the
adaptations between the organism and the outer world. But if this takes
place, where is there left any room for the action for his so-called
perfecting principle? Nägeli proceeds to show how he supposes that the
two work together.
As a result of inner causes the organism would pass through a series of
perfectly definite stages, J, J^1, J^2. But if, at any stage, external
influences produced an effect on the organism so that the arrangement of
the idioplasm changes in response, a new adaptation is produced. In this
way new characters, not inherent in the idioplasm, may be added, and old
ones be changed or lost. “In order not to be misunderstood in regard to
the completing or perfecting principle I will add, that I ascribe to it
no determinate action in the organism, neither in producing the long
neck of the giraffe, nor the prehensile tail of the ape, neither the
claws of the crab, nor the decoration of the bird of paradise. These
structures are the outcome of both factors. I cannot picture to myself
how external causes alone, and just as little how internal causes alone,
could have changed a monad into a man.” But Nägeli goes on to say, that
if at any stage of organization one of the two causes should cease to
act, the other could only produce certain limited results. Thus, if
external causes alone acted, the organization would remain at the same
stage of completeness, but might become adapted to all kinds of external
conditions—a worm, for instance, would not develop into a fish, but
would remain a worm forever, although it might change its worm structure
in many ways in response to external stimuli. If, on the other hand,
only the completing principle acted, then without changing its
adaptations the number of the cells and the size of the organs might be
increased, and functions that were formerly united might become
separated. Thus, without altering the character of the organism, a more
highly developed (in the sense of being more specialized) organism would
appear.
Nägeli, as we have just seen, has attempted to build up a conception of
nature based on two assumptions, neither of which has been demonstrated
to be an actual principle of development. His hypothesis appears,
therefore, entirely arbitrary and speculative to a high degree. Even if
it were conceivable that two such principles as these control the
evolution of organisms, it still requires a good deal of imagination to
conceive how the two go on working together. Moreover, it is highly
probable that whole groups have evolved in the direction of greater
simplification, as seen especially in the case of those groups that have
become degenerate. To what principle can we refer processes of this
sort?
It is certainly a strange conclusion this, at which Nägeli finally
arrives, for, after strenuously combating the idea that the external
factors of climate and of food have influence in producing new species,
he does not hesitate to ascribe all sorts of imaginary influences to
other external causes. The apparent contradiction is due, perhaps, to
the fact that his experience with actual species led him to deny that
the direct action of the environment produces permanent changes, while
in theory he saw the necessity of adding to his perfecting principle
some other factor to explain the adaptations of the new forms produced
by inner causes. Nägeli seems to have felt strongly the impossibility of
explaining the process of evolution and of adaptation as the outcome of
the selection of chance variations, now in this direction, now in that.
He seems to have felt that there must be something within the organism
that is driving it ever upward, and he attempts to avoid the
teleological element, which such a conception is almost certain to
introduce, by postulating the inheritance of the effects of
long-continued action of the environment, in so far as certain factors
in the environment produce a response in the organism. Nevertheless,
this combination is not one that is likely to commend itself, aside from
the fact that the assumptions have no evidence to support them. Despite
Nägeli’s protest that his principles are purely physical, and that there
is nothing mystical in his point of view, it must be admitted that his
conception, as a whole, is so vague and difficult in its application
that it probably deserves the neglect which it generally receives.
Nägeli’s wide experience with living plants convinced him that there is
something in the organism over and beyond the influence of the external
world that causes organisms to change; and we cannot afford, I think, to
despise his judgment on this point, although we need not follow him to
the length of supposing that this internal influence is a “force”
driving the organism forward in the direction of ever greater
complexity. A more moderate estimate would be that the organism often
changes through influences that appear to us to be internal, and while
some of the changes are merely fluctuating or chance variations, there
are others that appear to be more limited in number, but perfectly
definite and permanent in character. It is the latter, which, I believe,
we can safely accredit to internal factors, and which may be compared to
Nägeli’s internal causes, but this is far from assuming that these
changes are in the direction of greater completeness or perfection, or
that evolution would take place independently of the action of external
agencies.
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