existence within other animals, and how difficult and how much a matter
of chance it must be even to reach to such a place as, for instance,
the intestine, the liver, the lungs, or even the brain or the blood of
another animal, and when, on the other hand, we know how exactly things
are now regulated for every parasitic species so that its existence is
secured notwithstanding its dependence upon chance, we must undoubtedly
form a high estimate of the plasticity of the forms of life and their
adaptability. And this impression will only be strengthened when we
remember that the majority of internal parasites do not pass directly
from one host to another, but do so only through their descendants, and
that these descendants, too, must undergo the most far-reaching and
often unexpected adaptations in relation to their distribution, their
penetration into a new host, and their migrations and change of form
within it, if the existence of the species is to be secured.
We are tempted to study these relations more closely; but it is now
time to sum up, and we must no longer lose ourselves in wealth of
detail. Moreover, the life-history of many parasites, and of the
tape-worm in particular, is widely known, and any one can easily fill
up the story, of which we have given a mere outline. I simply wish to
point out that in parasitic animals there is a vast range of forms of
life in which the most precise adaptation to the conditions occurs
in almost every organ, and certainly at every stage of life, in the
most conspicuous and distinct manner. In the earlier part of these
lectures we gained from the study of the diverse protective means by
which plants and animals secure their existence the impression that
whatever is suited to its end (_Das Zweckmässige_) does not depend
upon chance for its origin, but that every adaptation which lies at
all within the possibilities of a species will arise if there is any
occasion for it. This impression is notably strengthened when we think
of the life-history of parasites, and we shall find that our view
of adaptations as arising, not through the selection of indefinite
variations, but through that of variations in a definite direction,
will be confirmed. Adaptations so diverse, and succeeding one another
in such an unfailing order as those in the life-history of a tape-worm,
a liver-fluke, or a _Sacculina_, cannot possibly depend upon pure
chance.
Nevertheless, chance does play a part in adaptations and
species-transformations, and that not only in relation to the
fundamental processes within the germ-plasm, but also in connexion
with the higher stages of the processes of selection, as I have
already briefly indicated. After the publication of my hypothesis of
germinal selection it was triumphantly pointed out that I had at last
been obliged to admit a phyletic evolutionary force, the 'definitely
directed' variation of Nägeli and Askenazy. This reproach--if to
allow oneself to be convinced be a reproach--is based upon a serious
misunderstanding. My 'variation in a definite direction' does not refer
to the evolution of the organic world as a whole. I do not suppose,
as Nägeli did, that this would have turned out essentially as it has
actually done, even although the conditions of life or their succession
upon the earth had been totally different; I believe that the organic
world, its classes and orders, its families and species, would have
differed from those that have actually existed, both in succession and
appearance, in proportion as the conditions of life were different.
My 'variation in a definite direction' is not predetermined from the
beginning, is not, so to speak, exclusive, but is many-sided; each
determinant of a germ-plasm may vary in a plus or minus direction, and
may continue under certain circumstances in the direction once begun,
but its components, the different biophors, may do the same, and so
likewise may the groups, larger and smaller, of biophors which form
the primordia (_Anlagen_) of the organs within the germ-plasm. Thus
an enormously large number of variational tendencies is available for
every part of the complete organism, and as soon as a variation would
be of advantage it arises--given that it is within the possibilities
of the physical constitution of the species. It occurs because its
potentialities are already present, but it persists and follows a
definite course because this is the one that is favoured. In other
words, it is primarily fixed by germinal selection alone, but is then
preferred by personal selection above the variants running parallel
with it. In my opinion the definite direction of the chance germinal
variations is determined only by the advantage which it affords to
the species with regard to its capacity for existence. But according
to Nägeli the direction of a variation is quite independent of its
utility, which may or may not exist. From Nägeli's point of view we
could never understand the all-prevailing adaptation, but if the
utility of a variant is itself sufficient to raise it to the level of a
persistent variational tendency, then we understand it.
Years ago (1883) I compared the species to a wanderer who has before
him a vast immeasurable land, through which he is at liberty to choose
whatever path he prefers, and in which he may sojourn wherever and
for as long as he pleases. But although he may go or stay entirely
of his own free will, yet at all times his going or staying will be
determined--it must be so and cannot be otherwise--by two factors:
first, by the paths available at each place--the variations which crop
up--and secondly, by the prospects each of these available paths open
up to him. He is striving after a restful place of abode which shall
afford him comfortable subsistence, his former home having been spoilt
for him by increasing expensiveness or too great competition. Even the
direction of his first journey will not depend upon chance, since of
the many paths available he will, and must, choose that which leads to
a habitable and not too crowded spot. If this has been reached--that is
to say, if the species has adapted itself to the new conditions--the
colonist sets up his abode there, and remains as long as a comfortable
existence and a competence are secure; but if these fail him, if grain
becomes scarce, or if prices rise, or if a dangerous epidemic breaks
out, then he makes up his mind to wander anew, and once more he will
choose, among the many available paths, that which offers him the
prospect of the speediest and most certain exit from the threatened
region, and leads him to another where he may live without risk. There,
too, he will remain as long as he is comfortable and not exposed to
want or danger, for the species as a whole only becomes transformed
when it must. And so it will go on _ad infinitum_; the traveller will,
when he is scared away from one dwelling-place, be able to continue his
journey in many directions, but he will always select the one path
which offers him the best prospects of a comfortable settlement, and
will follow it only to the nearest suitable place of abode, and never
further. The transformation of a species only goes on until it has
again completely adapted itself. In this way he will in the course of
years have traversed a large number of different places which, taken
together, may lie in a strange and unintelligible course, but this
course has nevertheless not arisen through a mere whim, but through
the twofold necessity of starting from a given spot--that in which he
had previously lived--the constitution of the species, and secondly of
choosing the most promising among the many available paths.
But chance does play a part in determining the route of the traveller,
for on it depends the nature of the conditions in the surroundings of
his previous dwelling-place, when he is forced to make another move;
for these conditions change, colonies are extended or depopulated,
a town previously cheap becomes dear, competition increases or
decreases, disease breaks out or disappears; in short, the chances of a
pleasureable sojourn in a particular place may alter and determine the
wanderer who is on the point of leaving his place of abode to take a
different direction from that which he would probably have chosen, say,
ten years earlier.
The analogy might be carried further, as, for instance, to illustrate
the possibility of a splitting up of the species; we may suppose that
instead of one wanderer there is a pair, who found a family at their
first halting-place. Children and grandchildren grow up in numbers and
food becomes scarce. One part of the descendants still finds enough
to live upon, but the rest set out to look for a new habitation. In
this case, too, many paths, sidewards or backwards, stand open to the
wanderers, but only those paths will be actually and successfully
followed by any company of them which will lead to a habitable place
where settlement is possible. If some of the descendants follow paths
with no such prospect they will soon turn back or will succumb to the
perils of the journey.
It seems to me that the contrast between this and Nägeli's view of
the transmutation of species is obvious enough. According to him the
wanderer is not free to choose his path, but goes on and on along a
definite railway-line that only diverges here and there, and it cannot
be foreseen whether the track leads to paradisaic dwellings or to
barren wastes--the travellers must just make the best of what they
find. They carry a marvellous travelling outfit with them--a sort of
_Tischlein, deck' dich_--the Lamarckian principle, but the magic power
of this is very doubtful, and it will hardly suffice to guard them
against the heat of the deserts, the frost of the Arctic regions, or
the malaria of the marshes into which their locomotive blindly carries
them.
According to my view, the traveler--that is, the species--has always
a large choice of paths, and is able, even while he is on the way,
to discern whether he has chosen a right or a wrong one; moreover,
in most cases, one or, it may be, a number of the paths lead to the
desired dwelling-place. But it also undoubtedly happens that, after
long wandering and when many regions have been traversed, a company
may finally arrive at a place which is quite habitable and inviting at
first sight, but which is surrounded on several sides by the sea or by
a rushing stream. As long as the soil remains fertile and the climate
healthy all goes well, but when matters change in this respect, and
perhaps the only way back lies through marshes and desert land and is
therefore impassable, then the colony will gradually die out--that is
the death of the species.
But let us now leave our parable and inquire what paths the organic
world has actually taken in its transformations, in what succession
the individual forms of life have evolved from one another; in short,
how the actual genealogical tree of this earth's animate population is
really constructed in detail. To this I can only reply that we have
many well-grounded suppositions, but only real certainty in regard
to isolated cases. Thus the genealogical tree of the horse has been
traced far back, and a great deal is known of the phylogeny of several
Gastropods and Cephalopods, but in regard to the genealogical tree
of organisms as a whole we can only make guesses, many of which are
probable, but are never quite certain. The palæontological records
which the earth's crust has preserved for us for all the ages are much
too incomplete to admit of any certainty. Many naturalists, notably
Ernst Haeckel, have done good service in this direction, for from
what we know of palæontology, embryology, and morphology, they have
constructed genealogical trees of the different groups of organisms,
which are intended to show us the actual succession of animal and plant
forms. But, interesting as these attempts are, they cannot for the
most part be anything more than guesswork, and I need not, therefore,
state or discuss them here in any detail, since they can afford us no
aid in regard to the problem of the origin of species with which these
lectures are concerned. In regard to the animal world at least--and the
case of plants is probably very similar--the record of fossil forms
fails us at an early stage. Thus the oldest and deepest strata in which
fossils can be demonstrated, the Cambrian formation, already contains
Crustaceans, animals at a relatively high stage of organization, which
must have been preceded by a very long series of ancestors of which
no trace has been preserved. The whole basal portion of the animal
genealogical tree, from the lowest forms of life at least up to these
primitive Crustaceans, the Trilobites, lies buried in the deepest
sedimentary rocks raised from the sea-floor, the crystalline schists,
in which it is unrecognizable. Enormous pressure and, probably also,
high temperature have destroyed the solid parts as far as there were
any, and the soft parts have only left an occasional impression even in
the higher strata.
Thus enormous periods of time must have elapsed from the beginning
of life to the laying down of that deepest 'Palæozoic' formation,
the Cambrian, for not only does the whole chain which leads from the
Biophoridæ to the origin of the first unicellulars fall within this
period, as well as the evolution of these unicellulars themselves
into their different classes, and their integration into the first
multicellulars, but also the evolution of these last into all the main
branches of the animal kingdom as it is now, into Sponges, Starfishes,
and their allies, Molluscs, Brachiopods, and Crustaceans, for all these
branches appear even in the Cambrian formation, and we may conclude
that the worms also, most of which are soft and not likely to be
preserved, were abundantly present at that time, since jointed animals
like the Crustaceans can only have arisen from worms. Moreover, we have
every reason for the assumption that Cœlenterates also, that is to say
polyps and medusoids, lived in the Cambrian seas, because their near
relatives with a solid skeleton, the corals, are represented in the
formation next above, the Silurian. The same is true of the fishes,
of which the first undoubtedly recognizable remains, the spines of
sharks, have been found in the Silurian. These two presuppose a long
preparatory history, and thus we come to the conclusion already stated,
that all the branches of the animal kingdom were already in existence
when the earth's crust shut up within itself the first records
available for us of the ancestors of our modern world of organisms.
Of course at that time the higher branches had only been represented
by their lower classes, and this is true especially of vertebrates,
so that, from the laying down of the Cambrian strata to the modern
world of organisms, a very considerable increase of complexity in
structure and an infinite diversifying of new groups must have taken
place. Amphibians do not appear to have been present in Cambrian times;
reptiles are represented in the Carboniferous strata, but only appear
in abundance in Secondary times; birds appear first in the Jurassic,
but in a very different guise (_Archæopteryx_) from the modern forms,
covered indeed with feathers, but still possessing a reptilian tail;
later they occur as toothed birds in the Cretaceous, and in Tertiary
times they have their present form. The development of mammals must
have run almost parallel with that of birds, that is, from the
beginning of Secondary times onwards, and their highest and last member
appears, as far as is known to research, only in post-Glacial times, in
the Diluvial deposits.
To the types which have arisen since the Cambrian period belongs the
class of Insects with its twelve orders and its enormous wealth of
known species, now reckoned at 200,000. They are demonstrable first in
the Devonian, and then in the Carboniferous period, in forms, just as
our theory requires, with _biting_ mouth-organs; it is not until the
Cretaceous strata that insects with purely suctorial mouth-organs--bees
and butterflies--occur, as it was also at that time that the flowers,
which have evolved in mutual adaptation with insects, first appeared.
The number of fossil species hitherto described is reckoned at about
80,000--certainly only a mere fragment of the wealth of forms of
life which have arisen on our earth throughout this long period, and
which must have passed away again; for very few _species_ outlive a
geological epoch, and even genera appear only for a longer or shorter
time, and then disappear for ever. But even of many of the older
classes, such, for instance, as the Cystoids among the Echinoderms of
the Silurian seas, no living representative remains; and in the same
way, the Ichthyosaurs or fish-lizards of the Secondary times have
completely disappeared from our modern fauna, and many other animal
types, like the class of Brachiopods and the hard-scaled Ganoid fishes,
have almost died out and are represented only by a few species in
specially sheltered places, such as the great depths of the sea, or in
rivers.
Thus an incredible wealth of animal and plant species was potentially
contained in these simplest and lowest 'Biophorids' which lay far
below the limits of microscopic visibility--an indefinitely greater
wealth than has actually arisen, for that is only a small part of
what was possible, and of what would have arisen had the changes of
life-conditions and life-possibilities followed a different course. The
greater the complexity of the structure of an organism is, the more
numerous are the parts of it which are capable of variation, and the
different directions in which it can adapt itself to new conditions;
and it will hardly be disputed that _potentially_ the first Biophorids
contained an absolutely inexhaustible wealth of forms of life, and
not merely those which have actually been evolved. If this were not
so, Man could not still call forth new animal and plant forms, as he
is continually doing among our domesticated animals and cultivated
plants, just as the chemist is continually 'creating' new combinations
in the laboratory which have probably never yet occurred or been
formed on the earth. But just as the chemist does not really 'create'
these combinations, but only brings the necessary elements and their
forces together in such a combination that they must unite to form the
desired new body, so the breeder only guides the variational tendencies
contained in the germ-plasm, and consciously combines them to procure a
new race. And what the breeder does within the narrow limits of human
power is being accomplished in free nature, through the conditions
which allow only what is fit to survive and reproduce, and thus bring
about the wonderful result--as though it were guided by a superior
intelligence--the adaptation of species to their environment.
Thus in our time the great riddle has been solved--the riddle of the
origin of what is suited to its purpose, without the co-operation of
purposive forces. Although we cannot demonstrate and follow out the
particular processes of transformation and adaptation in all their
phases with mathematical certainty, we can understand the principle,
and we see the factors through the co-operation of which the result
must be brought about. It has lately become the fashion, at least among
the younger school of biologists, to attach small value to natural
selection, if not, indeed, to regard it as a superseded formula;
mathematical proofs are demanded or, at any rate, desired. I do not
believe that we shall ever arrive at giving such proofs, but we shall
undoubtedly succeed in clearing up much that now remains obscure, and
in essentially modifying and correcting many of the theories we have
formed in regard to this question. But what has been already gained
must certainly be regarded as an enormous advance on the knowledge of
fifty years ago. We now _know_ that the modern world of organisms has
been evolved, and we can form an idea, though still only an imperfect
one, how and through the co-operation of what factors it could and must
have evolved.
When I say _must_, this refers only to the course of evolution from
a given beginning; but as to this beginning itself, the spontaneous
generation of the lowest Biophorids from inorganic material, we are far
from having understood it as a necessary outcome of its causes. And if
we have assumed it as a reasonable postulate, we by no means seek to
conceal that this assumption is far from implying an understanding of
what the process of biogenesis was. I do not merely mean that we do
not know under what external conditions the origin of living matter,
even in the smallest quantity, can take place; I mean, especially,
that we do not understand how this one substance should suddenly
reveal qualities which have never been detected in any other chemical
combination whatever--the circulation of matter, metabolism, growth,
sensation, will, and movement. But we may confidently say that we shall
never be able fully to understand these specific phenomena of life, as
indeed how should we, since nothing analogous to them is known to us,
and since understanding always presupposes a comparison with something
known. Even although we assume that we might succeed in understanding
the mere chemistry of life, as is not inconceivable, I mean the
_perpetuum mobile_ of dissimilation and assimilation, the so-called
'animal' functions of the living substance would remain uncomprehended:
Sensation, Will, Thought. We understand in some measure how the kidneys
secrete urine, or the liver bile; we can also--given the sensitiveness
to stimulus of the living substance--understand how a sense-impression
may be conveyed by the nerves to the brain, carried along certain
reflex paths to motor nerves and give rise to movement of the muscles,
but how the activity of certain brain-elements can give rise to a
thought _which cannot be compared with anything material_, which is
nevertheless able to react upon the material parts of our body, and, as
Will, to give rise to movement--that we attempt in vain to understand.
Of course the dependence of thinking and willing upon a material
substratum is clear enough, and it can be demonstrated with certainty
in many directions, and thus materialism is so far justified in drawing
parallels between the brain and thought on the one hand, and the
kidneys and urine on the other, but this is by no means to say that we
have understood how Thought and Will have come to be. In recent times
it has often been pointed out that the physical functions of the body
increase very gradually with the successive stages of the organization,
and from the lowest beginnings ascend slowly to the intelligence of
Man, in exact correspondence with the height of organization that has
been reached by the species; that they begin so imperceptibly among
the lower animal forms that we cannot tell exactly where the beginning
is; and it has been rightly concluded from this that the elements of
the Psyche do not originate in the histological parts of the nervous
system, but are peculiar to all living matter, and it has further been
inferred that even inorganic material may contain them, although in an
unrecognizable expression, and that their emergence in living matter
is, so to speak, only a phenomenon of summation. If we are right in our
assumption of a spontaneous generation it can hardly be otherwise, but
saying this does not mean that we have understood Spirit, but at most
secures us the advantage and the right of looking at this world, as far
as we know it, as a unity. This is the standpoint of Monism.