MINUTE AND GRADUAL MODIFICATIONS.
There are difficulties as to minute modifications, even if not
fortuitous.--Examples of sudden and considerable modifications of
different kinds.--Professor Owen's view.--Mr. Wallace.--Professor
Huxley.--Objections to sudden
changes.--Labyrinthodont.--Potto.--Cetacea.--As to origin of bird's
wing.--Tendrils of climbing plants.--Animals once supposed to be
connecting links.--Early specialization of
structure.--Macrauchenia.--Glyptodon.--Sabre-toothed
tiger.--Conclusion.
Not only are there good reasons against the acceptance of the exclusive
operation of "Natural Selection" as the one means of specific origination,
but there are difficulties in the way of accounting for such origination by
the sole action of modifications which are infinitesimal and minute,
whether fortuitous or not.
Arguments may yet be advanced in favour of the view that new species have
from time to time manifested themselves with suddenness, and by
modifications appearing at once (as great in degree as are those which
separate _Hipparion_ from _Equus_), the species remaining stable in the
intervals of such modifications: by stable being meant that their
variations only extend for a certain degree in various directions, like
oscillations in a stable equilibrium. This is the conception of Mr.
Galton,[76] who compares the development of species with a many {98}
facetted spheroid tumbling over from one facet, or stable equilibrium, to
another. The existence of internal conditions in animals corresponding with
such facets is denied by pure Darwinians, but it is contended in this work,
though not in this chapter, that something may also be said for their
existence.
The considerations brought forward in the last two chapters, namely, the
difficulties with regard to incipient and closely similar structures
respectively, together with palæontological considerations to be noticed
later, appear to point strongly in the direction of sudden and considerable
changes. This is notably the case as regards the young oysters already
mentioned, which were taken from the shores of England and placed in the
Mediterranean, and at once altered their mode of growth and formed
prominent diverging rays, _like those of the proper Mediterranean oyster_;
as also the twenty-nine kinds of American trees, all differing from their
nearest European allies _similarly_--"leaves less toothed, buds and seeds
smaller, fewer branchlets," &c. To these may be added other facts given by
Mr. Darwin. Thus he says, "that climate, to a certain extent, directly
modifies the form of dogs."[77]
The Rev. R. Everett found that setters at Delhi, though most carefully
paired, yet had young with "nostrils more contracted, noses more pointed,
size inferior, and limbs more slender." Again, cats at Mombas, on the coast
of Africa, have short stiff hairs instead of fur, and a cat at Algoa Bay,
when left only eight weeks at Mombas, "underwent a complete metamorphosis,
having parted with its sandy-coloured fur."[78] The conditions of life seem
to produce a considerable effect on horses, and instances are given by Mr.
Darwin of pony breeds[79] having independently arisen in different parts of
the world, possessing a certain similarity in their physical {99}
conditions. Also changes due to climate may be brought about at once in a
second generation, though no appreciable modification is shown by the
first. Thus "Sir Charles Lyell mentions that some Englishmen, engaged in
conducting the operations of the Real del Monte Company in Mexico, carried
out with them some greyhounds of the best breed to hunt the hares which
abound in that country. It was found that the greyhounds could not support
the fatigues of a long chase in this attenuated atmosphere, and before they
could come up with their prey they lay down gasping for breath; but these
same animals have produced whelps, which have grown up, and are not in the
least degree incommoded by the want of density in the air, but run down the
hares with as much ease as do the fleetest of their race in this
country."[80]
We have here no action of "Natural Selection;" it was not that certain
puppies happened accidentally to be capable of enduring more rarefied air,
and so survived, but the offspring were directly modified by the action of
surrounding conditions. Neither was the change elaborated by minute
modifications in many successive generations, but appeared at once in the
second.
With regard once more to sudden alterations of form, Nathusius is said to
state positively as to pigs,[81] that the result of common experience and
of his experiments was that rich and abundant food, given during youth,
tends by some direct action to make the head broader and shorter. Curious
jaw appendages often characterize Normandy pigs, according to M. Eudes
Deslongchamps. Richardson figures these appendages on the old "Irish
greyhound pig," and they are said by Nathusius to appear occasionally in
all the long-eared races. Mr. Darwin observes,[82] "As no wild pigs are
known to have analogous appendages, we have at present no reason to {100}
suppose that their appearance is due to reversion; and if this be so, we
are forced to admit that somewhat complex, though apparently useless
structures may be suddenly developed without the aid of selection." Again,
"Climate directly affects the thickness of the skin and hair" of
cattle.[83] In the English climate an individual Porto Santo rabbit[84]
recovered the proper colour of its fur in rather less than four years. The
effect of the climate of India on the turkey is considerable. Mr. Blyth[85]
describes it as being much degenerated in size, "utterly incapable of
rising on the wing," of a black colour, and "with long pendulous appendages
over the beak enormously developed." Mr. Darwin again tells us that there
has suddenly appeared in a bed of common broccoli a peculiar variety,
faithfully transmitting its newly acquired and remarkable characters;[86]
also that there have been a rapid transformation and transplantation of
American varieties of maize with a European variety;[87] that certainly
"the Ancon and Manchamp breeds of sheep," and that (all but certainly)
Niata cattle, turnspit and pug dogs, jumper and frizzled fowls, short-faced
tumbler pigeons, hook-billed ducks, &c., and a multitude of vegetable
varieties, have suddenly appeared in nearly the same state as we now see
them.[88] Lastly, Mr. Darwin tells us, that there has been an occasional
development (in five distinct cases) in England of the "japanned" or
"black-shouldered peacock" (_Pavo nigripennis_), a distinct species,
according to Dr. Sclater,[89] yet arising in Sir J. Trevelyan's flock
composed entirely of the common kind, and increasing, "_to the extinction
of the previously existing breed_."[90] Mr. Darwin's only explanation of
the phenomena (on the supposition of the species being distinct) is by{101}
reversion, owing to a supposed ancestral cross. But he candidly admits, "I
have heard of no other such case in the animal or vegetable kingdom." On
the supposition of its being only a variety, he observes, "The case is the
most remarkable ever recorded of the abrupt appearance of a new form, which
so closely resembles a true species, that it has deceived one of the most
experienced of living ornithologists."
As to plants, M. C. Naudin[91] has given the following instances of the
sudden origination of apparently permanent forms. "The first case mentioned
is that of a poppy, which took on a remarkable variation in its fruit--a
crown of secondary capsules being added to the normal central capsule. A
field of such poppies was grown, and M. Göppert, with seed from this field,
obtained still this monstrous form in great quantity. Deformities of ferns
are sometimes sought after by fern-growers. They are now always obtained by
taking spores from the abnormal parts of the monstrous fern; from which
spores ferns presenting the same peculiarities invariably grow.... The most
remarkable case is that observed by Dr. Godron, of Nancy. In 1861 that
botanist observed, amongst a sowing of _Datura tatula_, the fruits of which
are very spinous, a single individual of which the capsule was perfectly
smooth. The seeds taken from this plant all furnished plants having the
character of this individual. The fifth and sixth generations are now
growing without exhibiting the least tendency to revert to the spinous
form. More remarkable still, when crossed with the normal _Datura tatula_,
hybrids were produced, which, in the second generation, reverted to the
original types, as true hybrids do."
There are, then, abundant instances to prove that considerable {102}
modifications may suddenly develop themselves, either due to external
conditions or to obscure internal causes in the organisms which exhibit
them. Moreover, these modifications, from whatever cause arising, are
capable of reproduction--the modified individuals "breeding true."
The question is whether new species have been developed by non-fortuitous
variations which are insignificant and minute, or whether such variations
have been comparatively sudden, and of appreciable size and importance?
Either hypothesis will suit the views here maintained equally well (those
views being opposed only to fortuitous, indefinite variations), but the
latter is the more remote from the Darwinian conception, and yet has much
to be said in its favour.
Professor Owen considers, with regard to specific origination, that natural
history "teaches that the change would be sudden and considerable: it
opposes the idea that species are transmitted by minute and slow
degrees."[92] "An innate tendency to deviate from parental type, operating
through periods of adequate duration," being "the most probable nature, or
way of operation of the secondary law, whereby species have been derived
one from the other."[93]
Now, considering the number of instances adduced of sudden modifications in
domestic animals, it is somewhat startling to meet with Mr. Darwin's
dogmatic assertion that it is "a _false belief_" that natural species have
often originated in the same abrupt manner. The belief _may_ be false, but
it is difficult to see how its falsehood can be positively asserted.
It is demonstrated by Mr. Darwin's careful weighings and measurements,
that, though little used parts in domestic animals get reduced in weight
and somewhat in size, yet that they show no inclination to become truly
"rudimentary structures." Accordingly he asserts[94] that such {103}
rudimentary parts are formed "suddenly, by arrest of development" in
domesticated animals, but in wild animals slowly. The latter assertion,
however, is a _mere assertion_; necessary, perhaps, for the theory of
"Natural Selection," but as yet unproved by facts.
But why should not these changes take place suddenly in a state of nature?
As Mr. Murphy says,[95] "It may be true that we have no evidence of the
origin of wild species in this way. But this is not a case in which
negative evidence proves anything. We have never witnessed the origin of a
wild species by any process whatever; and if a species were to come
suddenly into being in the wild state, as the Ancon Sheep did under
domestication, how could you ascertain the fact? If the first of a
newly-begotten species were found, the fact of its discovery would tell
nothing about its origin. Naturalists would register it as a very rare
species, having been only once met with, but they would have no means of
knowing whether it were the first or the last of its race."
To this Mr. Wallace has replied (in his review of Mr. Murphy's work in
_Nature_[96]), by objecting that sudden changes could very rarely be
useful, because each kind of animal is a nicely balanced and adjusted
whole, any one sudden modification of which would in most cases be hurtful
unless accompanied by other simultaneous and harmonious modifications. If,
however, it is not unlikely that there is an innate tendency to deviate at
certain times, and under certain conditions, it is no more unlikely that
that innate tendency should be an harmonious one, calculated to
simultaneously adjust the various parts of the organism to their new
relations. The objection as to the sudden abortion of rudimentary organs
may be similarly met.
Professor Huxley seems now disposed to accept the, at least {104}
occasional, intervention of sudden and considerable variations. In his
review of Professor Kölliker's[97] criticisms, he himself says,[98] "We
greatly suspect that she" (_i.e._ Nature) "does make considerable jumps in
the way of variation now and then, and that these saltations give rise to
some of the gaps which appear to exist in the series of known forms."
[Illustration: MUCH ENLARGED HORIZONTAL SECTION OF THE TOOTH OF A
LABYRINTHODON.]
In addition to the instances brought forward in the second chapter against
the minute action of Natural Selection, may be mentioned such {105}
structures as the wonderfully folded teeth of the labyrinthodonts. The
marvellously complex structure of these organs is not merely unaccountable
as due to Natural "Selection," but its production by insignificant
increments of complexity is hardly less difficult to comprehend.
Similarly the aborted index of the Potto (_Perodicticus_) is a structure
not likely to have been induced by minute changes; while, as to "Natural
Selection," the reduction of the fore-finger to a mere rudiment is
inexplicable indeed! "How this mutilation can have aided in the struggle
for life, we must confess, baffles our conjectures on the subject; for that
any very appreciable gain to the individual can have resulted from the
slightly lessened degree of required nourishment thence resulting (_i.e._
from the suppression), seems to us to be an almost absurd proposition."[99]
[Illustration: HAND OF THE POTTO (PERODICTICUS), FROM LIFE.]
Again, to anticipate somewhat, the great group of whales (Cetacea) was
fully developed at the deposition of the Eocene strata. On the other hand,
we may pretty safely conclude that these animals were absent as late as the
latest secondary rocks, so that their development could not have been so
very slow, unless geological time is (although we shall presently see there
are grounds to believe it is not) practically infinite. It is quite true
that it is, in general, very unsafe to infer the absence of any animal
forms during a certain geological period, because no remains of them {106}
have as yet been found in the strata then deposited: but in the case of the
Cetacea it is safe to do so; for, as Sir Charles Lyell remarks,[100] they
are animals, the remains of which are singularly likely to have been
preserved had they existed, in the same way that the remains were preserved
of the Ichthyosauri and Plesiosauri, which appear to have represented the
Cetacea during the secondary geological period.
[Illustration: SKELETON OF A PLESIOSAURUS.]
As another example, let us take the origin of wings, such as exist in
birds. Here we find an arm, the bones of the hand of which are atrophied
and reduced in number, as compared with those of most other Vertebrates.
Now, if the wing arose from a terrestrial or subaërial organ, this abortion
of the bones could hardly have been serviceable--hardly have preserved
individuals in the struggle for life. If it arose from an aquatic organ,
like the wing of the penguin, we have then a singular divergence from the
ordinary vertebrate fin-limb. In the ichthyosaurus, in the plesiosaurus, in
the whales, in the porpoises, in the seals, and in others, we have
shortening of the bones, but no reduction in the number either of the
fingers or of their joints, which are, on the contrary, multiplied in
Cetacea and the ichthyosaurus. And even in the turtles we have eight carpal
bones and five digits, while no finger has less than two phalanges. It{107}
is difficult, then, to believe that the Avian limb was developed in any
other way than by a comparatively sudden modification of a marked and
important kind.
[Illustration: SKELETON OF AN ICHTHYOSAURUS.]
How, once more, can we conceive the peculiar actions of the tendrils of
some climbing plants to have been produced by minute modifications? These,
according to Mr. Darwin,[101] oscillate till they touch an object, and then
embrace it. It is stated by that observer, "that a thread weighing no more
than the thirty-second of a grain, if placed on the tendril of the
_Passiflora gracilis_, will cause it to bend; and merely to touch the
tendril with a twig causes it to bend; but if the twig is at once removed,
the tendril soon straightens itself. But the contact of other tendrils of
the plant, or of the falling of drops of rain, do not produce these
effects."[102] But some of the zoological and anatomical discoveries of
late years tend rather to diminish than to augment the evidence in favour
of minute and gradual modification. Thus all naturalists now admit that
certain animals, which were at one time supposed to be connecting links
between groups, belong altogether to one group, and not at all to the
other. For example, the aye-aye[103] (_Chiromys Madagascariensis_). {108}
was till lately considered to be allied to the squirrels, and was often
classed with them in the rodent order, principally on account of its
dentition; at the same time that its affinities to the lemurs and apes were
admitted. The thorough investigation into its anatomy that has now been
made, demonstrates that it has no more essential affinity to rodents than
any other lemurine creature has.
[Illustration: THE AYE-AYE.]
Bats were, by the earliest observers, naturally supposed to have a close
relationship to birds, and cetaceans to fishes. It is almost superfluous to
observe that all now agree that these mammals make not even an approach to
either one or other of the two inferior classes.
{109}
In the same way it has been recently supposed that those extinct flying
saurians, the pterodactyles, had an affinity with birds more marked than
any other known animals. Now, however, as has been said earlier, it is
contended that not only had they no such close affinity, but that other
extinct reptiles had a far closer one.
The _amphibia_ (_i.e._ frogs, toads, and efts) were long considered (and
are so still by some) to be reptiles, showing an affinity to fishes. It now
appears that they form with the latter one great group--the ichthyopsida of
Professor Huxley--which differs widely from reptiles; while its two
component classes (fishes and amphibians) are difficult to separate from
each other in a thoroughly satisfactory manner.
If we admit the hypothesis of gradual and minute modification, the
succession of organisms on this planet must have been a progress from the
more general to the more special, and no doubt this has been the case in
the majority of instances. Yet it cannot be denied that some of the most
recently formed fossils show a structure singularly more generalized than
any exhibited by older forms; while others are more specialized than are
any allied creatures of the existing creation.
A notable example of the former circumstance is offered by macrauchenia--a
hoofed animal, which was at first supposed to be a kind of great llama
(whence its name)--the llama being a ruminant, which, like all the rest,
has two toes to each foot. Now hoofed animals are divisible into two very
distinct series, according as the number of functional toes on each hind
foot is odd or even. And many other characters are found to go with this
obvious one. Even the very earliest Ungulata show this distinction, which
is completely developed and marked even in the Eocene palæotherium and
anoplotherium found in Paris by Cuvier. The former of these has the toes
odd (perissodactyle), the other has them even (artiodactyle).
Now, the macrauchenia, from the first relics of it which were found, {110}
was thought to belong, as has been said, to the even-toed division.
Subsequent discoveries, however, seemed to give it an equal claim to rank
amongst the perissodactyle forms. Others again inclined the balance of
probability towards the artiodactyle. Finally, it appears that this very
recently extinct beast presents a highly generalized type of structure,
uniting in one organic form both artiodactyle and perissodactyle
characters, and that in a manner not similarly found in any other known
creature living, or fossil. At the same time the differentiation of
artiodactyle and perissodactyle forms existed as long ago as in the period
of the Eocene ungulata, and that in a marked degree, as has been before
observed.
Again, no armadillo _now living_ presents nearly so remarkable a speciality
of structure as was possessed by the _extinct_ glyptodon. In that singular
animal the spinal column had most of its joints fused together, forming a
rigid cylindrical rod, a modification, as far as yet known, absolutely
peculiar to it.
[Illustration: DENTITION OF THE SABRE-TOOTHED TIGER (MACHAIRODUS).]
In a similar way the _extinct_ machairodus, or sabre-toothed tiger, is
characterized by a more highly differentiated and specially carnivorous
dentition than is shown by any predacious beast of the _present day_. {111}
The specialization is of this kind. The grinding teeth (or molars) of
beasts are divided into premolars and true molars. The premolars are molars
which have deciduous vertical predecessors (or milk teeth), and any which
are in front of such, _i.e._ between such and the canine tooth. The true
molars are those placed behind the molars having deciduous vertical
predecessors. Now, as a dentition becomes more distinctly carnivorous, so
the hindmost molars and the foremost premolars disappear. In the existing
cats this process is carried so far that in the upper jaw only one true
molar is left on each side. In the machairodus there is no upper true molar
at all, while the premolars are reduced to two, there being only these two
teeth above, on each side, behind the canine.
Now, with regard to these instances of early specialization, as also with
regard to the changed estimate of the degrees of affinity between forms, it
is not pretended for a moment that such facts are irreconcilable with
"Natural Selection." Nevertheless, they point in an opposite direction. Of
course not only is it conceivable that certain antique types arrived at a
high degree of specialization and then disappeared; but it is manifest they
did do so. Still the fact of this early degree of excessive specialization
tells to a certain, however small, extent against a progress through
excessively minute steps, whether fortuitous or not; as also does the
distinctness of forms formerly supposed to constitute connecting links.
For, it must not be forgotten, that if species have manifested themselves
generally by gradual and minute modifications, then the absence, not in one
but in _all cases_, of such connecting links, is a phenomenon which remains
to be accounted for.
It appears then that, apart from fortuitous changes, there are certain
difficulties in the way of accepting extremely minute modifications of any
kind, although these difficulties may not be insuperable. Something, at all
events, is to be said in favour of the opinion that sudden and {112}
appreciable changes have from time to time occurred, however they may have
been induced. Marked _races_ have undoubtedly so arisen (some striking
instances having been here recorded), and it is at least conceivable that
such may be the mode of _specific_ manifestation generally, the possible
conditions as to which will be considered in a later chapter. [Page 113]
* * * * *