by Emery, that excessive variation in the same direction from
intra-germinal causes has not rarely been the cause of the extinction
of species. I also mentioned the very similar view of Döderlein,
who could not refer at that time to germinal selection, but assumed
internal compelling forces, which pressed a variation irresistibly
forward in the direction in which it had started, even beyond the
bounds of what is useful for the desired end, and which might thus
bring about the extinction of the species. I cannot entirely agree
with these views, as I have already indicated, because I do not
believe that the impulse to variation can ever become irresistible
and uncontrollable. If it could, then we should not see, as we do,
innumerable cases in which the augmentation or diminution of a part has
gone on precisely to the point at which it ceases to be purposeful.
Even the degeneration of organs only proceeds as far as is necessary
to accomplish a particular end, as we see plainly from the parasitic
Crustaceans of different orders. In many of these parasitic forms the
swimming legs degenerate, but in the female only, because these attach
themselves by suckers or in some other manner to their host, so that
they cannot let go again. But the males need their swimming legs to
seek out the females. The females too require them in their youth,
in order to seek out the fish from which they are to obtain their
food-supply, and thus the degeneration of the swimming legs has come to
a full stop exactly at the point where they cease to be of use; they
develop in early youth and degenerate later, when the animal becomes
sessile. In accordance with the law of biogenesis we may say that
while the degeneration is complete in the final stages of ontogeny,
its retrogression was not continued back to the germ, but only to the
young stages. From this it follows that the progress of a variation
may at any time have a goal fixed for it, and we have seen that this
is possible by means of personal selection, which accumulates the
never-failing fluctuations of the variation in the direction of plus or
of minus. In the individual id a determinant _X_ may perhaps decrease
and possibly also increase without limit, although we have no certain
knowledge in regard to the latter point, but as this determinant is
contained in all the ids, there are always plus and minus fluctuations
by means of which personal selection can operate.
But of course it requires a certain amount of time for this, and in
the fact that this time is often not available lies, I think, the
reason why excessive differentiations have often led to the extinction
of a species, not because the increase of the excessive organ must go
on irresistibly, but because changes in the conditions have made the
exuberant organ inappropriate, and it could not degenerate quickly
enough to save the species from extinction.
Brandes has recently given a beautiful illustration of this by
associating the existence of the remarkable sabre-toothed tigers
(_Machairodus_) with enormously long canine teeth, which lived in the
Diluvial period in South America, with the gigantic Armadillos which
lived there at the same time, whose bony armature two yards in height
now excites our admiration. He rightly points out that the dentition of
_Machairodus neogæus_ is by no means a typically perfect dentition for
a beast of prey, like that of the Indian tiger or the lion; as far as
incisors and molars are concerned it was much less effective than that
of these predatory animals, and the great length of the dagger-like
flattened canines, which protruded far beyond the mouth, entirely
prevented the bringing together of the teeth of the upper and lower jaw
after the fashion of a pair of pincers. He rightly infers from this
that this dentition was adapted to a specialized mode of nutrition, and
he regards the great mailed Armadillos, such as the three-yards-long
heavy Glyptodont of the Pampas, as the victims into which they were
wont to thrust their sabre-teeth in the region of the unprotected
neck, and thus to master the almost invulnerable creature, which was
invincible as far as all other predatory animals were concerned. Thus
the remarkable dentition is explained on the one hand, and on the other
the amazing extent and hardness of the victim's coat of mail. Thus,
too, we can understand why there should have been at that time a whole
series of cat-like animals with sabre-like teeth, in which the length
and sharpness of the teeth increased with the bodily size, for these
predatory animals corresponded to a whole series of Armadillos, whose
size was increasing, as was also the strength of their armour.
Of course this interpretation is hypothetical, but it contains much
internal probability, so that it may be taken as a good illustration
of the reciprocal increase of adaptations between two animal groups.
We understand now why, on the one hand, this colossal tortoise-like
armour should have developed in a mammal, and, on the other hand,
why these enormously long sabre-teeth should have been evolved; we
also understand--and this is the point with which we are here chiefly
concerned--why these two 'excessive' developments should ultimately
lead to the destruction of their possessors. For a long period
the Armadillos were able to save themselves from extermination by
increasing their bodily size and the strength of their armour, and
they thus saved themselves from persecution on the part of beasts of
prey with smaller and weaker teeth. But the predatory animals followed
suit and lengthened their teeth and increased their bodily size,
until ultimately even the strongest armour of the victim afforded
no efficient protection, and the mighty Glyptodonts were by degrees
utterly exterminated. But then the death-knell of the _Machairodus_
had also sounded, for he was so exactly adapted to this one kind of
diet that he could no longer overpower other victims and feed on their
flesh; the sabre-teeth prevented him from tearing his prey like other
predatory animals, he could probably only suck them.
Even if this is a supposititious case, it serves to show that it was
not an internal principle of variation that caused the teeth of these
carnivores and the armour of their victims to increase so unlimitedly;
it was the necessity of adaptation. They did not perish because armour
and teeth increased so excessively, but because neither of these
adaptations could be neutralized all at once, and small variations were
of no use to them in their final struggle for survival.
In a certain sense we may say that simpler, more lowly organisms are
more capable of adaptation than those which are highly differentiated
and adapted to specialized conditions in all parts of their bodies,
since from the former much that is new may arise in the course of time,
while very little and nothing very novel can spring from the latter.
From the simplest Protozoan the whole world of unicellular organisms
could arise, and also the much more diverse Metazoa; from the lower
marine worms there could arise not only many kinds of higher marine
worms--the segmented worms or Annelids--but also quite new groups of
animals, the Arthropods and the Vertebrates. It is hardly likely that a
new class of animals will evolve from our modern birds, because these
are already so perfectly adapted to their aerial life that they could
hardly adapt themselves to life on land or in the water sufficiently
well to be able to hold their own in regard to all the possibilities of
life with the rest of the dwellers on land or in water. We do indeed
know of birds which have returned entirely to a purely terrestrial
life--the ostriches, for instance--and of others which have adapted
themselves to a purely aquatic life, such as the penguins, but these
are small groups of species, and are hardly likely to increase. On
the contrary, we can prove that many have already succumbed in the
struggle with man, and we anticipate the extermination of others. But
the reason why they are so readily exterminated obviously lies in the
fact that they have surrendered the advantage given to them by their
bird-nature, by adapting themselves to terrestrial life, and that they
are not able to regain it, at least not in the short time that is at
their disposal if they are to be saved from extermination. The best
example of this is the Dodo (_Didus ineptus_). This remarkable-looking
bird, of about the size of a swan, lived in flocks upon the island of
Mauritius until about the end of the seventeenth century. It had small
wings with short quills which were useless for flight. As it could
neither escape by flight nor through the water, and could only move
clumsily and awkwardly upon land with its short legs and heavy body, it
was hopelessly doomed as soon as a stronger enemy made his appearance.
It fell a victim to the sailors who first landed on the island and
clubbed it with sticks in huge numbers. Until that event it was without
doubt excellently adapted to life on that fertile island, for on a
volcanic island in the middle of the ocean there were no large enemies,
and it was therefore not dependent on the power of flight for safety,
and could pick up abundant food from the ground. But when man suddenly
appeared on the scene and began to persecute it, it was not the 'senile
rigidity' of its organization that prevented it from making use of
its wings again; it was the slowness of variation and consequently
of selection, which is common to all species, which impelled it to
extinction. The same fate will probably overtake the Kiwi of New
Zealand (_Apteryx australis_) in the near future, for though it has
so far escaped the arrows of the aborigines, it is not likely in its
wingless condition to be able to hold out long against European guns,
unless close times and preserved forests are instituted for it, as they
have been for our chamois.
Even sadder from the biologist's point of view than such extermination
of individual species through the vandalism and greed of our own
race is the disturbance of whole societies of animals and plants by
man that is going on or has been accomplished on most of the oceanic
islands, and we must briefly notice these cases while we are dealing
with the decadence of species. I refer to the crowding out of the
usually endemic animal and plant population on such islands through
cultivation. The first step in this work of 'cultivation' is always the
cutting down of the forests which for thousands of years have clothed
these islands as with a mantle of green, have regulated their rainfall,
secured their fertility, and allowed a medley of indigenous animals,
usually peculiar to the spot, to arise. We have already spoken of St.
Helena. The original and remarkable fauna and flora of this island had
for the most part disappeared 200 years ago, through the cutting down
of trees in the forests, and these were later wholly destroyed by the
introduction of goats, which devoured all the young trees as they grew.
But with the forests most of the indigenous insects and birds were
doomed to destruction, so that now there is not an indigenous bird or
butterfly to be found there; only a few terrestrial snails and beetles
of the original fauna still survive.
But it is not only on islands that a large number of species have
been decimated or entirely exterminated by deforestation, by the
introduction of plants cultivated by man and of the 'weeds' associated
with these, and by the importation of domesticated animals. In Central
Europe not only have all the larger beasts of prey, like the bear, the
lynx, and the wolf, almost completely disappeared, but the reindeer,
the bison, the wild ox (Aurochs), and the elk have been exterminated
as wild animals, and in North America the buffalo will soon only exist
in preserved herds, if that is not already the case. Here, of course,
the direct interference of the all-too-powerful enemy, man, has played
the largest part in causing the disappearance of the species referred
to, but the process may give us an idea of the way in which a superior
animal enemy may be able gradually to exterminate a weaker species
where there is no attainable or even conceivable variation which
might preserve them from such a fate. Several of the mammals which I
have mentioned are not yet entirely exterminated; even the Aurochs
perhaps still exists in the pure white herds preserved in some British
parks; but there are more instances than that of the Dodo of the utter
extermination of a species through human agency within historic times.
It may be doubtful whether the sea-otter (_Enhydris marina_) has not
been already quite exterminated because of its precious fur, but it is
quite certain that the huge sea-cow (_Rhytina stelleri_), which lived
in large numbers in the Behring Straits at the end of the eighteenth
and the beginning of the nineteenth centuries, was completely
exterminated by sailors within a few decades.
We may therefore gain from what is going on before our eyes, so to
speak, some sort of idea of the way in which the extermination of
species may go on even independently of man at the present time, and
how it must have gone on also in past ages of the earth's history.
Migrations of species have taken place ceaselessly, although very
slowly, for every species is endeavouring slowly to extend its range
and to take possession of new territories, and thus the fauna and
flora of any region must have changed in the course of time, new
species must have settled in it from time to time, and the conditions
of life must have changed, and in many cases this must have led to the
extermination of species, in the same way, though not so quickly, as
human interference now brings about their doom.
This is true for plants as for animals. A good example, not indeed of
complete extermination, but of very considerable diminution in the
numbers of individuals of a plant-species by the advent of a species of
mammal, is communicated to us by Chun in regard to Kerguelen Land. A
flowering plant, the Kerguelen cabbage (_Pringlea antiscorbutica_), has
been greatly reduced in numbers since the thoughtless introduction of
rabbits to this uninhabited island (1874). While, in 1840, Captain Ross
used this plant in great quantities as a preventative against scurvy in
his crew, and even carried away stores to last for months, the Valdivia
Expedition in 1898 found rabbits in abundance, but the Kerguelen
cabbage had been entirely exterminated at every spot accessible to
these prolific and voracious rodents. It was only found growing upon
perpendicular cliffs or upon the islands lying out in the fiords.
An avoidance of the threatened destruction of a species by its
adaptation to the new circumstances can only be possible when the
changes occur very slowly, and will therefore be more likely to be
achieved in the case of physical changes in the conditions of life,
such as climatic changes, a change in the mutual relations of land
and sea, and so on. But it appears that even climatic changes do
not evoke any variation and new adaptation as long as the species
can avoid the changes by migrating. The often quoted case of Alpine
and Arctic plants proves this at any rate, that those species which
inhabited the plateaus and highlands of Europe did not all vary to
suit the change when a warmer climate prevailed, but that in part at
least they followed the climate to which they were already adapted,
that is, that they migrated towards the north on the one hand and
higher up the Alps on the other. It cannot be denied that many of the
insects and plants did adapt themselves at that time to the warmer
climate, and became the modern species which now inhabit the plains,
for many related species occur on the Alps and in the plains, but
apparently many others simply made their escape from a climate which
no longer suited their requirements. Thus, as far as I am aware, there
is no species of _Primula_ in South or Central Germany which could be
derived from the beautiful red _Primula farinosa_ of the Alps, but this
species occurs also upon the old glacier-soil at the northern base of
the Alps, and in similar soil again in the north of Germany and on the
meadows of Holstein. Similar examples might be cited in regard to the
Alpine-Arctic butterflies.
It is intelligible enough that we are still very far from being able
to give a precise account of the main changes in the plant and animal
world during the history of the earth in regard to the special causes
which have produced them. Possibly the future will throw more light
upon this subject by extending our knowledge of the fossil remains
of all countries. But so much at least we can say at present, that
there is no reason to refer the dying out of the earlier forms to
anything else than the changes in the conditions of life, the struggle
for existence, and the limitation of the power of transformation and
adaptation due to the organization which the species has already
attained; there is no trace of any such thing as a phyletic principle
of life in the vitalistic sense, as far as the decadence of species is
concerned.
LECTURE XXXVI
SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION
Spontaneous generation--Experimental tests impossible--Only the
lowest and smallest forms of life can be referred to spontaneous
generation--Chemical postulates for spontaneous generation--Empedocles
modernized--The locality of spontaneous generation--Progress of
organization--Direct and indirect influences causing variation--The
various modes of selection--Everything depends upon selection--Sinking
from heights of organization already attained--Paths of evolution--The
forces effecting it--Plasticity of living matter--Predetermination
of the animate world--Many-sided adaptation of each group--Aquatic
mammals and insects, parasites--Nägeli's variation in a definite
direction--Analogy of the traveller--Genealogical trees--The
diversity of forms of life is unlimited--The origin of the purposeful
apart from purposive forces working towards an end--The limits of
knowledge--Limitation of the human intelligence by selection--Human
genius--Conclusion.
We have now reached the end of our studies, and they have given us
satisfaction, at least in so far that they have brought us certainty
in regard to the chief and fundamental question which can be asked in
reference to the origin of the modern animate world of organisms. There
remains no doubt in our minds that the theory of descent is justified;
we know, just as surely as that the earth goes round the sun, that the
living world upon our earth was not created all at once and in the
state in which we know it, but that it has gradually evolved through
what, to our human estimate, seem enormously long periods of time.
This conclusion is now firmly established and will never again become
doubtful. The assumption, too, that the more lowly organisms formed
the beginnings of life, and that an ascent has taken place from the
lowest to the higher and highest, has become to our minds a probability
verging upon certainty. But there remains one point which we have not
yet touched upon--the problem of the origin of these first organisms.
There are only two possibilities: either that they have been borne to
our earth from outside, from somewhere else in the universe, or that
they have originated upon our earth itself through what is called
'spontaneous generation'--_generatio spontanea_.
The idea that very lowly living organisms might have been concealed
within the clefts and crevices of meteorites, and might thus have
fallen upon our earth and so have formed the first germs of life, was
first formulated by that chemical genius, Justus Liebig. It seems
certain that the state of glowing heat in which meteorites are, when
they come into our atmosphere, only affects the outer crust of these
cosmic fragments, and that living germs, which might be concealed in
the depths of their crevices and fissures, might therefore remain
alive, but nevertheless it is undoubtedly impossible that any germ
should reach us alive in this way, because it could neither endure
the excessive cold nor the absolute desiccation to which it would be
exposed in cosmic space, which contains absolutely no water. This could
not be endured even for a few days, much less for immeasurable periods
of time.
But we have to take account, too, of an entirely general reason,
which lies in the fact that all life is transient, that it can be
annihilated, and is not merely mortal! Everything that is distinctively
organic may be destroyed to the extent of becoming inorganic. Not
only may the phenomena of life disappear, and the living body as such
cease to be, but the organic compounds which form the physical basis
of all life are ceaselessly breaking up, and they fall back by stages
to the level of the inorganic. It seems to me that we must necessarily
conclude from this that the basis of Liebig's idea was incorrect, that
is, the assumption that 'organic substances are everlasting and have
existed from the first just in the same way as inorganic substances.'
This is obviously not the case, for a thing that has an end cannot be
everlasting; it must have had a beginning too, and consequently organic
combinations are not everlasting, but are transitory; they come and
go, they arise wherever the conditions suitable for them occur, and
they break up into simpler combinations when these conditions cease
to be present. It is only the elements which are eternal, not their
combinations, for these are subject to more or less rapid continual
change, whether they have arisen outside of organisms or within them.
It seems to me that these considerations destroy the foundations of
the hypothesis of the cosmic origin of life on our earth; in any case
they leave the hypothesis without great significance; for if we could
even admit the possibility of a transference of living organisms from
space, the question would only be pushed a little further back by the
assumption, and not solved, for the organisms thus brought in must have
had their origin on some other planet, since they are, _ex hypothesi_,
not everlasting.
Thus we are directed to our earth itself as the place of the origin
of the tellurian world of life, and I see no possibility of avoiding
the assumption of _spontaneous generation_. It is for me a logical
necessity.
Even about the middle of the nineteenth century there was acute
discussion in regard to the occurrence of spontaneous generation. In
the French Academy especially Pouchet brought forward arguments in
favour of it, and Pasteur against it. Pouchet observed that living
organisms made their appearance in infusions of hay and other vegetable
material in which any possible living germs had presumably been
destroyed by prolonged boiling. Living organisms, Algæ, and Infusorians
appeared, notwithstanding the fact that the glass bottles in which
they were kept were hermetically sealed. But Pasteur showed that the
air contains numerous living germs of lowly organisms in its so-called
motes, and that, if these were first removed, Pouchet's infusion would
not exhibit any signs of life. He caused the air, which was continually
passed through the tubes, to stream first along the heated barrel of
a gun, and so destroyed these germs, and no organisms were obtained
in the infusions. He showed that the air is teeming with germs by an
experiment with boiled infusions which were allowed to lie undisturbed
for a considerable time in bottles with open necks, one on the roof
of the Institute at Paris, the other on the top of the Puy de Dôme in
Auvergne, which was at that time still the highest mountain in France.
In the Parisian experiment, organisms appeared in the bottles in a very
few days, while in those exposed to the pure air at the mountain-top
none were seen, even after months had elapsed.
Strangely enough, these and similar experiments were at the time
regarded as conclusive proof against the existence of spontaneous
generation, though it is obvious enough that the first living being
on this earth cannot have sprung from hay, or from any other organic
substance, since that would presuppose what we are attempting to
explain. After the fiery earth had so far cooled that its outermost
layer had hardened to a firm crust, and after water had condensed to a
liquid form, there could at first only have been inorganic substances
in existence. In order to prove spontaneous generation, therefore,
it would be necessary to try to find out from what mingling of
inorganic combinations organisms could arise; to prove that spontaneous
generation could never have been possible is out of the question.
It would be impossible to prove by experiment that spontaneous
generation could _never_ have taken place; because each negative
experiment would only prove that life does not arise _under the
conditions of the experiment_. But this by no means excludes the
possibility that it might arise under other conditions.
Up till now all attempts to discover these conditions have been futile,
and I do not believe that they will ever be successful, not because the
conditions must be so peculiar in nature that we cannot reproduce them,
but above all, because we should not be able to perceive the results
of a successful experiment. I shall be able to prove this convincingly
without difficulty.