LAWS OF TRANSMISSION BY INHERITANCE. ADAPTATION AND NUTRITION.
Distinction between Conservative and Progressive
Transmission by Inheritance.—Laws of Conservative
Transmission: Transmission of Inherited
Characters.—Uninterrupted or Continuous
Transmission.—Interrupted or Latent
Transmission.—Alternation of
Generations.—Relapse.—Degeneracy.—Sexual
Transmission.—Secondary Sexual Characters.—Mixed or
Amphigonous Transmission.—Hybrids.—Abridged or
Simplified Transmission.—Laws of Progressive
Inheritance: Transmission of Acquired
Characters.—Adapted or Acquired Transmission.—Fixed or
Established Transmission.—Homochronous Transmission
(Identity in Epoch).—Homotopic Transmission (Identity in
Part).—Adaptation and Mutability.—Connection between
Adaptation and Nutrition.—Distinction between Indirect
and Direct Adaptation.
In the last chapter we considered Transmission by Inheritance, one of
the two universal vital activities of organisms, Adaptation and
Inheritance, which by their interaction produce the different species of
organisms, and we have endeavoured to trace this very mysterious vital
activity to a more general physiological function of organisms, namely,
to Propagation. This latter in its turn, like other vital phenomena of
animals and plants, depends on physical and chemical relations. It is
true they appear at times exceedingly complicated, but can nevertheless
in reality be traced to simple mechanical causes—that is, to the
relations of attraction and repulsion in the particles or molecules—in
fact, to the motional phenomena of matter.
Now, before we turn our attention to the second function, the phenomenon
of Adaptation or Mutability, which counteracts the Transmission by
Inheritance, it seems appropriate first to cast one more glance at the
various manifestations of Heredity, which we may perhaps even now
denominate the “_laws of transmission by inheritance._” Unfortunately,
up to the present time very little has been done for this most important
subject, either in zoology or in botany, and almost all we know of the
different laws of inheritance is confined to the experiences of
gardeners and farmers. It is not therefore to be wondered at, that on
the whole these exceedingly interesting and important phenomena have not
been investigated with desirable scientific accuracy, or reduced to the
form of scientific laws. Accordingly, what I shall relate of the
different laws of transmission are only some preliminary fragments taken
out of the infinitely rich store which lies open to our inquiry.
We may first divide all the different phenomena of inheritance into two
groups, which we may distinguish as the transmission of _inherited_
characters, and the transmission of _acquired_ characters; and we may
call the former the _conservative_ transmission, and the latter the
_progressive_ transmission by inheritance. This distinction depends upon
the exceedingly important fact that the individuals of every species of
animals and plants can transmit to their descendants, not only those
qualities which they themselves have inherited from their ancestors, but
also the peculiar, individual qualities which they have acquired during
their own life. The latter are transmitted by progressive, the former
by conservative inheritance. We have now first to examine the phenomena
of _conservative inheritance_, that is, the transmission of such
qualities as the organism has already received from its parents or
ancestors. (Gen. Morph. ii. 180.)
Among the phenomena of conservative inheritance we are first struck by
that which is its most general law, and which we may term the _law of
uninterrupted or continuous transmission_. It is so universal among the
higher animals and plants, that the uninitiated might overestimate its
action and consider it as the only normal law of transmission by
inheritance. This law simply consists in the fact that among most
species of animals and plants, every generation is, on the whole, like
the preceding—that the parents are as like the grandparents as they are
like the children. “Like produces like,” as is commonly said, but more
accurately “similar things produce similar things.” For, in reality, the
descendants of every organism are never absolutely equal in all points,
but only similar in a greater or less degree. This law is so generally
known, that I need not give any examples of it.
The _law of interrupted or latent transmission_ by inheritance, which
might also be termed alternating transmission, is in a measure opposed
to the preceding law. This important law appears principally active
among many lower animals and plants, and manifests itself in contrast to
the former in the fact that the offspring are not like their parents,
but very dissimilar, and that only the third or a later generation
becomes similar to the first. The grandchildren are like the
grandparents, but quite unlike the parents. This is a remarkable
phenomenon, and, as is well known, occurs also very frequently, though
in a less degree, in human families. Every one of my readers doubtless
knows some members of a family who, in this or that peculiarity, much
more resemble the grandfather or grandmother than the father or mother.
Sometimes it lies in bodily peculiarities, for example, features of
face, colour of hair, size of body—sometimes in mental qualities, for
example, temperament, energy, understanding—which are transmitted in
this manner. This fact may be observed in domestic animals as well as in
the case of man. Among the domestic animals most liable to vary—as the
dog, horse, and ox—breeders very frequently find that the product by
breeding resembles the grandparents far more than it does its own
parental organism. If we express this general law and the succession of
generations by the letters of the alphabet, then A = C = E, whilst
B = D = F, and so on.
This very remarkable fact appears in a more striking way in the lower
animals and plants than in the higher, and especially in the well-known
phenomenon of _alternation of generations_ (metagenesis). Here we very
frequently find—for example, among the Planarian worms, sea-squirts or
Tunicates, Zoophytes, and also among ferns and mosses—that the organic
individual in the first place produces, by propagation, a form
completely different from the parental form, and that only the
descendants of this generation, again, become like the first. This
regular change of generation was discovered by the poet Chamisso, on his
voyage round the world in 1819, among the _Salpæ_, cylindrical
tunicates, transparent like glass, which float on the surface of the
sea. Here the larger generation, the individuals of which live isolated
and possess an eye of the form of a horse-shoe, produce in a non-sexual
manner (by the formation of buds) a completely different and smaller
generation. The individuals of this second smaller generation live
united in chains and possess a cone-shaped eye. Every individual of such
a chain produces, in a sexual manner (hermaphrodite) again, a non-sexual
solitary form of the first and larger generation. Among the Salpæ,
therefore, it is always the first, third, and fifth generation, and in
like manner the second, fourth, and sixth generations, that are entirely
like one another. However, it is not always only one, but in other cases
a number of generations, which are thus leapt over; so that the first
generation resembles the fourth and seventh, the second resembles the
fifth and eighth, the third resembles the sixth and ninth, and so on.
Three different generations alternate with one another; for example,
among the neat _little sea-buoys_ (Doliolum), small tunicates closely
related to the Salpæ. In this case it is A = D = G, further, B = E = H,
and C = F = I. Among the plant-lice (Aphides), each sexual generation is
followed by a succession of from eight to ten or twelve non-sexual
generations, which are like one another, but differ from the sexual
generations. Then, again, a sexual generation reappears like the one
long before vanished.
If we further follow this remarkable law of latent or interrupted
inheritance, and take into consideration all the phenomena appertaining
to it, we may comprise under it also the well-known phenomena of
_reversion_. By the term “reversion” or “atavism” we understand the
remarkable fact known to all breeders of animals, that occasionally
single and individual animals assume a form which has not existed for
many generations, but belongs to a generation which has long since
disappeared. One of the most remarkable instances of this kind is the
fact that in some horses there sometimes appear singular dark stripes,
similar to those of the zebra, quagga, and other wild species of African
horses. Domestic horses of the most different races and of all colours
sometimes show such dark stripes; for example, a stripe along the back,
a stripe across the shoulders, and the like. The sudden appearance of
these stripes can only be explained by the supposition that it is the
effect of a latent transmission, a relapse into the ancient original
form, which has long since vanished, and was once common to all species
of horses; the original form, undoubtedly, was originally striped like
the zebras, quaggas, etc. In like manner, certain qualities in other
domestic animals sometimes appear quite suddenly, which once marked
their wild ancestors, now long since extinct. In plants, also, such a
relapse can be observed very frequently. All my readers probably know
the wild yellow toad-flax (Linaria vulgaris), a plant very common in our
fields and hedges. Its dragon-mouthed yellow flower contains two long
and two short stamens. But sometimes there appears a single blossom
(Peloria) which is funnel-shaped, and quite regularly composed of five
individual and equal sections, with five corresponding stamens. This
Peloria can only be explained as a relapse into the long since extinct
and very ancient common form of all those plants which, like the
toad-flax, possess dragon-mouthed, two-lipped flowers, with two long and
two short stamens. The original form, like the Peloria, possessed a
regular five-spurred blossom, with five equal stamens, which only later
and by degrees have become unequal (compare p. 17). All such relapses
are to be brought under the law of interrupted or latent transmission,
although the number of intervening generations may be enormous.
When cultivated plants or domestic animals become wild, when they are
withdrawn from the conditions of cultivated life, they experience
changes which appear not only as adaptations to their new mode of life,
but partially also as relapses into the ancient original form out of
which the cultivated forms have been developed. Thus the different kinds
of cabbage, which are exceedingly different in form, may be led back to
the original form, by allowing them to grow wild. In like manner, dogs,
horses, heifers, etc., when growing wild, often revert more or less to a
long extinct generation. An immensely long succession of generations may
pass away before this power of latent transmission becomes extinguished.
A third law of conservative transmission may be called the _law of
sexual transmission_, according to which each sex transmits to the
descendants of the same sex peculiarities which are not inherited by the
descendants of the other sex. The so-called secondary sexual characters,
which in many respects are of extraordinary interest, everywhere furnish
numerous examples of this law. Subordinate or secondary sexual
characters are those peculiarities of one of the two sexes which are not
directly connected with the sexual organs themselves; such characters,
which exclusively belong to the male sex, are, for example, the antlers
of the stag, the mane of the lion, and the spur of the cock. The human
beard, an ornament commonly denied to the female sex, belongs to the
same class. Similar characteristics by which the female sex is alone
distinguished are, for example, the developed breasts, with the
lactatory glands of female mammals and the pouch of the female opossum.
The bodily size, also, and complexion, differs in female animals of many
species from that of the male. All these secondary sexual qualities,
like the sexual organs themselves, are transmitted by the male organism
only to the male, not to the female, and _vice versâ_. Contrary facts
are rare exceptions to the rule.
A fourth law of transmission, which has here to be mentioned, in a
certain sense contradicts the last, and limits it, viz., the _law of
mixed or mutual_ (amphigonous) _transmission_. This law tells us that
every organic individual produced in a sexual way receives qualities
from both parents, from the father as well as from the mother. This
fact, that personal qualities of each of the two sexes are transmitted
to both male and female descendants, is very important, Goethe mentions
it of himself, in the beautiful lines—
“Von Vater hab ich die Statur, des Lebens ernstes Führen
Von Mütterchen die Frohnatur und Lust zu fabuliren.”
“From my father I have my stature and the serious tenour of my life,
From my mother a joyous nature and a turn for poetizing.”
This phenomenon, I suppose, is so well-known to all, that I need not
here enter upon it. It is according to the different portions of their
character which father and mother transmit to their children, that the
individual differences among brothers and sisters are chiefly
determined.
The very important and interesting phenomenon of _hybridism_ also
belongs to this law of mixed or amphigonous transmission. It alone,
when rightly estimated, is quite sufficient to refute the prevailing
dogma of the constancy of species. Plants, as well as animals, belonging
to quite different species, may sexually mingle with one another and
produce descendants which in many cases can again propagate themselves,
and that indeed either (more frequently) by mingling with one of the two
parental species, or (more rarely) by pure in-breeding, hybrid mixing
with hybrid. The latter is well established, for example, in the hybrids
of hares and rabbits (Lepus Darwinii, p. 147). The hybrids of a horse
and a donkey, two different species of the same genus (Equus), are well
known. These hybrids differ according as the father or the mother
belongs to the one or the other species—the horse or the donkey. The
mule produced by a mare and a he-donkey has qualities quite different
from those of the jinny (Hinnus), the hybrid of a horse and she-donkey.
In both cases the hybrid produced by the crossing of two different
species is a mixed form, which receives qualities from both parents; but
the qualities of the hybrid are different, according to the form of the
crossing. In like manner, mulattoes produced by a European and a negress
show a different mixture of characters from the hybrids produced by a
negro with a European female. In these phenomena of hybrid-breeding, as
well as in the other laws of transmission previously mentioned, we are
as yet unable to show the acting causes in detail; but no naturalist
doubts the fact that the causes are in all cases purely mechanical and
dependent upon the nature of organic matter itself. If we possessed more
delicate means of investigation than our rude organs of sense and
auxilliary instruments, we should be able to discover those causes, and
to trace them to the chemical and physical properties of matter.
Among the phenomena of conservative transmission, we must now mention,
as the fifth law, the _law of abridged or simplified transmission_. This
law is very important in regard to embryology or ontogeny, that is in
regard to the history of the development of organic individuals.
_Ontogeny_, or the history of the development of individuals, as I have
already mentioned in the first chapter (p. 10), and as I subsequently
shall explain more minutely, is nothing but a short and quick repetition
of _Phylogeny_ dependent on the laws of transmission and
adaptation—that is, a repetition of the palæontological history of
development of the whole organic tribe, or phylum, to which the organism
belongs. If, for example, we follow the individual development of a man,
an ape, or any other higher mammal within the maternal body from the
egg, we find that the fœtus or embryo arising out of the egg passes
through a series of very different forms, which on the whole agrees
with, or at least runs parallel to, a series of forms which is presented
to us by the historical chain of ancestors of the higher mammals. Among
these ancestors we may mention certain fishes, amphibians, marsupials,
etc. But the parallelism or agreement of these two series of development
is never quite complete; on the contrary, in ontogeny there are always
gaps and leaps which indicate the omission of certain stages belonging
to the phylogeny. Fritz Müller, in his excellent work, “Für Darwin,”(16)
has clearly shown in the case of the Crustacea, or crabs, that “the
historical record preserved in the individual history of development is
gradually obscured, in proportion as development takes a more and more
direct route from the egg to the complete animal.” This process of
obscuring and shortening is determined by the law of abridged
transmission, and I mention it here specially because it is of great
importance for the understanding of embryology, and because it explains
the fact, at first so strange, that the whole series of forms which our
ancestors have passed through in their gradual development are no longer
visible in the series of forms of our own individual development from
the egg.
Opposed to the laws of the conservative transmission, hitherto
discussed, are the phenomena of the transmission of the second series,
that is, the _laws of progressive transmission by inheritance_. As
already mentioned, they depend upon the fact that the organism transmits
to its descendants not only those qualities which it has inherited from
its own ancestors, but also a number of those individual qualities which
it has acquired during its own lifetime. Adaptation is here seen to be
connected with transmission by inheritance (Gen. Morph. ii. 186).
At the head of these important phenomena of progressive transmission, we
may mention the _law of adapted or acquired transmission_. In reality it
asserts nothing more than what I have said above, that in certain
circumstances the organism is capable of transmitting to its descendants
all the qualities which it has acquired during its own life by
adaptation. This phenomenon, of course, shows itself most distinctly
when the newly acquired peculiarity produces any considerable change in
the inherited form. This is the case in the examples I mentioned in the
preceding chapter as to transmission in general, in the case of the men
with six fingers and toes, the porcupine men, copper beeches, weeping
willows, etc. The transmission of acquired diseases, such as
consumption, madness, and albinism, likewise form very striking
examples. Albinoes are those individuals who are distinguished by the
absence of colouring matter, or pigments, in the skin. They are of
frequent occurrence among men, animals, and plants. In the case of
animals of a definite dark colour, individuals are not unfrequently born
which are entirely without colour, and in animals possessing eyes, this
absence of pigment extends even to the eyes, so that the iris of the
eye, which is commonly of a bright or intense colour, is colourless, but
appears red, on account of the blood-vessels being seen through it.
Among many animals, such as rabbits and mice, albinoes with white fur
and red eyes are so much liked that they are propagated in great numbers
as a special race. This would be impossible were it not for the law of
the transmission of adaptations.
Which of the changes acquired by an organism are transmitted to its
descendants, and which are not, cannot be determined _à priori_, and we
are unfortunately not acquainted with the definite conditions under
which the transmission takes place. We only know in a general way that
certain acquired qualities are much more easily transmitted than others,
for example, more easily than the mutilations caused by accidents. These
latter are generally not transmitted by inheritance, otherwise the
descendants of men who have lost their arms or legs would be born
without the corresponding arm or leg; but here, also, exceptions occur,
and a race of dogs without tails has been produced by consistently
cutting off the tails of both sexes of the dog during several
generations. A few years ago a case occurred on an estate near Jena, in
which by a careless slamming of a stable door the tail of a bull was
wrenched off, and the calves begotten by this bull were all born without
a tail. This is certainly an exception; but it is very important to note
the fact, that under certain unknown conditions such violent changes are
transmitted in the same manner as many diseases.
In very many cases the change which is transmitted and preserved by
adapted transmission is constitutional or inborn, as in the case of
albinism mentioned before. The change then depends upon that form of
adaptation which we call the indirect or potential. A very striking
instance is furnished by the hornless cattle of Paraguay, in South
America. A special race of oxen is there bred which is entirely without
horns. It is descended from a single bull, which was born in 1770 of an
ordinary pair of parents, and the absence of horns was the result of
some unknown cause. All the descendants of this bull produced with a
horned cow were entirely without horns. This quality was found
advantageous, and by propagating the hornless cattle among one another,
a hornless race was obtained, which at present has almost entirely
supplanted the horned cattle in Paraguay. The case of the otter-sheep of
North America forms a similar example. In the year 1791 a farmer, by
name Seth Wright, lived in Massachusetts, in North America; in his
normally formed flock of sheep a lamb was suddenly born with a
surprisingly long body and very short and crooked legs. It was therefore
unable to take any great leaps, and especially unable to leap across a
hedge into a neighbour’s garden—a quality which seemed advantageous to
the owner, as the territories were divided by hedges. It therefore
occurred to him to transmit this quality to other sheep, and by
crossing this ram with normally shaped ewes, he produced a whole race
of sheep, all of which had the qualities of the father, short and
crooked legs and a long body. None of them could leap across the hedges,
and they therefore were much liked and propagated in Massachusetts.
A second law, which likewise belongs to the series of progressive
transmissions, may be called the _law of established or habitual
transmission_. It manifests itself in this, that qualities acquired by
an organism during its individual life are the more certainly
transmitted to its descendants the longer the causes of that change have
been in action, and that this change becomes the more certainly the
property of all subsequent generations the longer the cause of change
acts upon these latter also. The quality newly acquired by adaptation or
mutation must be established or constituted to a certain degree before
we can calculate with any probability that it will be transmitted at all
to the descendants. In this respect transmission resembles adaptation.
The longer a newly acquired quality has been transmitted by inheritance,
the more certainly will it be preserved in future generations. If,
therefore, for example, a gardener by methodical treatment has produced
a new kind of apple, he may calculate with the greater certainty upon
preserving the desired peculiarity of this sort the longer he has
transmitted the same by inheritance. The same is clearly shown in the
transmission of diseases. The longer consumption or madness has been
hereditary in a family the deeper is the root of the evil, and the more
probable it is that all succeeding generations will suffer from it.
We may conclude the consideration of the phenomena of inheritance with
the two very important laws of _homotopic_ and _contemporaneous
transmission by inheritance_. We understand by them the fact that
changes acquired by an organism during its life, and transmitted to its
descendants, appear in the same part of the body in which the parental
organism was first affected by them, and that they also appear in the
offspring at the same age as that at which they did so in the parent.
_The law of contemporaneous or homochronous transmission_, which Darwin
calls the law of “transmission in corresponding periods of life,” can
be shown very clearly in the transmission of diseases, especially of
such as are recognized as very destructive, on account of their
hereditary character. They generally appear in the organism of the child
at the time corresponding with that in which the parental organism
contracted the disease. Hereditary diseases of the lungs, liver, teeth,
brain, skin, etc., usually appear in the descendants at the same period,
or a little earlier than they showed themselves in the parental
organism, or were contracted by it. The calf gets its horns at the same
period of life as its parents did. In like manner the young stag
receives its antlers at the same period of life in which they appeared
in its father or grandfather. In every one of the different sorts of
vine the grapes ripen at the same time as they did in the case of their
progenitors. It is well known that the time of ripening varies greatly
in the different sorts; but as all are descended from a single species,
this variation has been acquired by the progenitors of the several
sorts, and has then been transmitted by inheritance.
The _law of homotopic transmission_, which is most closely connected
with the last mentioned law, and which might be called the law of
transmission in corresponding parts of the body, may also be very
distinctly recognized in pathological cases of inheritance. Large moles,
for example, or accumulations of pigment in several parts of the skin,
tumours also, often appear during many generations, not only at the same
period of life, but also in the same part of the skin. Excessive
development of fat in certain parts of the body is likewise transmitted
by inheritance. Above all, it is to be noted that numerous examples of
this, as well as of the preceding law, may be found everywhere in the
study of embryology. Both the _law of homochronous and homotopic
transmission are fundamental laws of embryology, or ontogeny_. For these
laws explain the remarkable fact that the different successive forms of
individual development in all generations of one and the same species
always appear in the same order of succession, and that the variations
of the body always take place in the same parts. This apparently simple
and self-evident phenomenon is nevertheless exceedingly wonderful and
curious; we cannot explain its real causes, but may confidently assert
that they are due to the direct transmission of the organic matter from
the parental organism to that of the offspring, as we have seen above in
the case of the process of transmission in general, by a consideration
of the details of the various modes of reproduction.
Having thus, then, considered the most important laws of Inheritance, we
now turn to the second series of phenomena bearing on natural selection,
viz., to those of Adaptation or Variation. These phenomena, taken as a
whole, stand in a certain opposition to the phenomena of Inheritance,
and the difficulty which arises in examining them consists mainly in
the two sets of phenomena being so completely intercrossed and
interwoven. We are but seldom able to say with certainty—of the
variations of form which occur before our eyes—how much is owing to
Inheritance, and how much to Adaptation. All characters of form, by
which organisms are distinguished, are caused _either_ by Inheritance or
by Adaptation; but as both functions are continually interacting with
each other, it is extremely difficult for the systematic inquirer to
recognize the share belonging to each of the two functions in the
special structure of individual forms. This is, at present, all the more
difficult, because we are as yet scarcely aware of the immense
importance of this fact, and because most naturalists have neglected the
theory of Adaptation, as well as that of Inheritance. The laws of
Inheritance, which we have just discussed, as well as the laws of
Adaptation, which we shall consider directly, in reality form only a
small portion of the phenomena existing in this domain, but which have
not as yet been investigated; and since every one of these laws can
interact with every other, it is clear that there is an infinite
complication of physiological actions, which are at work in the
construction of organisms.
But now, as to the phenomenon of variation or adaptation in general, we
must, as in the case of inheritance, view it as a quite universal,
physiological fundamental quality of all organisms, without
exception—as a manifestation of life which cannot be separated from the
idea of organism. Strictly speaking, we must here also, as in the case
of inheritance, distinguish between Adaptation itself and Adaptability.
By Adaptation (Adaptio), or Variation (Variatio), we understand the fact
that the organism, in consequence of influences of the surrounding
outer world, assumes certain new peculiarities in its vital activity,
composition, and form which it has not inherited from its parents; these
acquired individual qualities are opposed to those which have been
inherited, or, in other words, those which have been transmitted to it
from its parents or ancestors. On the other hand, we call Adaptability
(Adaptabilitas), or Variability (Variabilitas), the capability inherent
in all organisms to acquire such new qualities under the influence of
the outer world. (Gen. Morph. ii. 191.)
The undeniable fact of organic adaptation or variation is universally
known, and can be observed at every moment in thousands of phenomena
surrounding us. But just because the phenomena of variation by external
influences appear so self-evident, they have hitherto undergone scarcely
any accurate scientific investigation. To them belong all the phenomena
which we look upon as the results of contracting and giving up habits,
of practice and giving up practices, or as the results of training, of
education, of acclimatization, of gymnastics, etc. Many permanent
variations brought about by causes producing disease, that is to say,
many diseases, are nothing but dangerous adaptations of the organism to
injurious conditions of life. In the case of cultivated plants and
domestic animals, variation is so striking and powerful that the breeder
of animals and the gardener found their whole mode of proceeding upon
it, or rather upon the interaction between these phenomena and those of
Inheritance. It is also well known to every one that animals and plants,
in their wild state, are subject to variation. Every systematic treatise
on a group of animals or plants, if it were to be quite complete and
exhaustive, ought to mention in every individual species the number of
variations which differ more or less from the prevailing or typical form
of the species. Indeed, in every careful systematic special treatise one
finds, in the case of most species, mention of a number of such
variations, which are described sometimes as individual deviations, and
sometimes as so-called races, varieties, degenerate species, or
subordinate species, and which often differ exceedingly from the
original species, solely in consequence of the adaptation of the
organism to the external conditions of life.
If we now endeavour to fathom the general causes of these phenomena of
Adaptation, we arrive at the conclusion that in reality they are as
simple as the causes of the phenomena of Inheritance. We have shown that
the nature of the process of propagation furnishes the real explanation
of the facts of Transmission by Inheritance, that is, the transmission
of parental matter to the body of the offspring; and in like manner we
can show that the physiological function of _nutrition_, or _change of
substance_, affords a general explanation of Adaptation or Variation.
When I here point to “nutrition” as the fundamental cause of variation
and adaptation, I take this word in its widest sense, and I understand
by it the whole of the material changes which the organism undergoes in
all its parts through the influences of the surrounding outer world.
Nutrition thus comprises not only the reception of actual nutritive
substances and the influence of different kinds of food, but also, for
example, the action upon the organism of water and of the atmosphere,
the influence of sunlight, of temperature, and of all those
meteorological phenomena which are implied in the term “climate.” The
indirect and direct influence of the nature of the soil and of the
dwelling-place also belong to it; and further, the extremely important
and varied influence which is exercised upon every animal and every
plant by the surrounding organisms, friends and neighbours, enemies and
robbers, parasites, etc. All these and many other very important
influences, all of which more or less modify the organism in its
material composition, must be taken into consideration in studying the
change of substance which goes on in living things. Adaptation,
accordingly, is the consequence of all those material variations which
are produced in the change of substance of the organism by the external
conditions of existence, or by the influences of the surrounding
external world.
How very much every organism is dependent upon the whole of its external
surroundings, and changed by their alteration, is, in a general way,
well known to every one. Only think how much the human power of action
is dependent upon the temperature of the air, or how much the
disposition of our minds depends upon the colour of the sky. Accordingly
as the sky is cloudless and sunny, or covered with large heavy clouds,
our state of mind is cheerful or dull. How differently do we feel and
think in a forest during a stormy winter night and during a bright
summer day! All the different moods of our soul depend upon purely
material changes of our brain, upon movements of molecular plasma, which
are started through the medium of the senses by the different influences
of light, warmth, moisture, etc. “We are a plaything to every pressure
of the air.” No less important and deeply influential are the effects
produced upon our mind and body by the different quality and quantity
of food. Our mental activity, the activity of our understanding and of
our imagination, is quite different accordingly as we have taken tea or
coffee, wine or beer, before or during our work. Our moods, wishes, and
feelings are quite different when we are hungry and when we are
satisfied. The national character of Englishmen and Gauchos, in South
America, who live principally on meat and food rich in nitrogen, is
wholly different from that of the Irish, feeding on potatoes, and that
of the Chinese, living on rice, both of whom take food deficient in
nitrogen. The latter also form much more fat than the former. Here, as
everywhere, the variations of the mind go hand in hand with the
corresponding transformations of the body; both are produced by purely
material causes. But all other organisms, in the same way as man, are
varied and changed by the different influences of nutrition. It is well
known that we can change in an arbitrary way the form, size, colour,
etc., of our cultivated plants and domestic animals, by change of food;
that, for example, we can take from or give to a plant definite
qualities, accordingly as we expose it to a greater or less degree of
sunlight and moisture. As these phenomena are generally widely known,
and as we shall proceed presently to the consideration of the different
laws of adaptation, we will not dwell here any longer on the general
facts of variation.
As the different laws of transmission may be naturally divided into the
two series of conservative and progressive transmission, so we may also
distinguish between two series of the laws of adaptation, first, the
series of laws of _indirect_, and secondly, the series of laws of
_direct_ adaptation. The latter may also be called the laws of actual,
and the former the laws of potential, adaptation.
The first series, comprising the phenomena of _indirect_ (potential)
adaptation, has, on the whole, hitherto been little attended to, and
Darwin has the merit of having directed special attention to this series
of changes. It is somewhat difficult to place this subject clearly
before the reader; I will endeavour to make it clear hereafter by
examples. Speaking quite generally, indirect or potential adaptation
consists in the fact that certain changes in the organism, effected by
the influence of nutrition (in its widest sense) and of the external
conditions of existence in general, show themselves not in the
individual form of the respective organism, but in that of its
descendants. Thus, especially in organisms propagating themselves in a
sexual way, the reproductive system, or sexual apparatus, is often
influenced by external causes (which little affect the rest of the
organism), to such a degree that its descendants show a complete
alteration of form. This can be seen very strikingly in artificially
produced monstrosities. Monstrosities can be produced by subjecting the
parental organism to certain extraordinary conditions of life, and,
curiously enough, such an extraordinary condition of life does not
produce a change of the organism itself, but a change in its
descendants. This cannot be called transmission by inheritance, because
it is not a quality existing in the parental organism that is
transmitted by inheritance. It is, on the contrary, a change affecting
the parental organism, but not perceptible in it, that appears in the
peculiar formation of its descendants. It is only the impulse to this
new formation which is transmitted in propagation through the egg of the
mother or the sperm of the father. The new formation exists in the
parental organism only as a possibility (potential); in the descendants
it becomes a reality (actual).
As this very important and very general phenomenon had hitherto been
entirely neglected, people were inclined to consider all the visible
variations and transformations of organic forms as phenomena of
adaptation of the second series, that is, as phenomena of _direct_ or
actual adaptation. The essence of this latter kind of adaptation
consists in the fact that the change affecting the organism (through
nutrition, etc.) shows itself immediately by some transformation, and
does not only make itself apparent in the descendants. To this class
belong all the well-known phenomena in which we can directly trace the
transforming influence of climate, food, education, training, etc., in
their effects upon the individual itself.
We have seen how the two series of phenomena of progressive and
conservative transmission, in spite of their difference in principle, in
many ways interfere with and modify each other, and in many ways
co-operate with and cross each other. The same is the case, in a still
higher degree, in the two series of phenomena of indirect and direct
adaptation, which are opposed to each other and yet closely connected.
Some naturalists, especially Darwin and Carl Vogt, ascribe to the
indirect or potential adaptation by far the more important and almost
exclusive influence. But the majority of naturalists have hitherto been
inclined to take the opposite view, and to attribute the principal
influence to direct or actual adaptation. I consider this controversy,
in the mean while, as almost useless. It is but seldom that we are in a
condition, in any individual case of variation, to judge how much of it
belongs to direct and how much to indirect adaptation. We are, on the
whole, still too little acquainted with these exceedingly important and
intricate relations, and can only assert, in a general way, that the
transformation of organic forms is to be ascribed _either_ to direct
adaptation alone, _or_ to indirect adaptation alone, or lastly, to the
co-operation of both direct _and_ indirect adaptation.