Henry Edward Crampton - The Doctrine of Evolution

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The Lamarckian explanation encounters one grave difficulty which is not
met by the second one, in so far as it demands some method by which a
bodily change may be introduced into the stream of inheritance. So far,
this difficulty has not been overcome, and the present verdict of science
is that the transmission of characters acquired as the result of other
than congenital factors is not proved. It would be unscientific to say
that it cannot be proved in the future, but there are good _a priori_
grounds for disbelief in the principle, while furthermore the results of
experiments that have been undertaken to test its truth have been entirely
negative. Rats and mice have had their tails cut off to see if this
mutilation would have its effect upon their young, and though this has
been done for more than one hundred successive generations the length of
the tail has not been altered. Quite unconscious of the scientific
problem, many human races have performed precisely similar experiments
through centuries of time. In some classes of Chinese, the feet of young
girls have been bound in such a way as to produce a small, malformed foot,
but this has not resulted in any hereditary diminution in the size of the
feet of Chinese females. Many other similar mutilations have been
practised, as for example, the flattening of the skull of some North
American Indians, but the deformity must be produced again with each
recurring generation. One after another, the cases that were supposed to
give positive evidence have been reinvestigated, with the result that has
been stated above. It would seem, therefore, that heredity and congenital
modification must play by far the greater part in the evolution of
species.

* * * * *

The doctrine of natural selection took form in the mind of Darwin mainly
on account of three potent influences; these were, first, the geological
doctrine of uniformitarianism proposed by Lyell, second, his own
observations of wild life in many lands and his analysis of the breeder's
results with domesticated animals, and third, the writings of Malthus
dealing with overpopulation. As Darwin had read the works of Buffon,
Lamarck, and Erasmus Darwin, his grandfather, who had written a famous
treatise under the title of "Zoonomia," he was familiar with the evidences
known in his student days tending to prove that organic evolution was a
real natural process. Lyell's doctrine of uniform geological history made
an early and deep impression upon his mind, and it led him to ask himself
whether the efficient causes of past evolution might not be revealed by an
analysis of the present workings of nature. As naturalist of the "Beagle"
during its four years' cruise around the world, Darwin saw many new lands
and observed varied circumstances under which the organisms of the tropics
and other regions lived their lives. The fierce struggle for existence
waged by the denizens of the jungle recalled to him the views of Malthus
regarding overpopulation and its results. These and other influences led
him to begin the remarkable series of note-books, from which it is
interesting indeed to learn how the doctrine of natural selection began to
assume a definite and permanent form in his mind, as year followed year,
and evidence was added to evidence. And it is a valuable lesson to the
student of science that for twenty-five years Darwin devoted all his time
to the acquisition of facts before he gave his doctrine to the world in
the famous "Origin of Species."

Darwin was particularly impressed by the way mankind has dealt with the
various species of domesticated animals, and he was the first naturalist
to point out the correspondence between the breeder's method of
"artificial selection," and the world-wide process of natural selection.
As every one knows, the breeder of race horses finds that colts vary much
in their speed; discarding the slower animals, he uses only the swifter
for breeding purposes, and so he perfects one type of horse. With other
objects in view, the heavy draught horse, the spirited hackney, and the
agile polo pony have been severally bred by exactly the same method. Among
cattle many kinds occur, again the products of an artificial or human
selection; hornless breeds have been originated, as well as others with
wide-spreading or sharply curved horns; the Holstein has been bred for an
abundant supply of milk as an object, while Jerseys and Alderneys excel in
the rich quality of their milk. Various kinds of domesticated sheep and
rabbits and cats also owe their existence to the employment of the
selfsame method, unconsciously copied by man from nature; for men have
found variations arising naturally among their domesticated animals, and
they have simply substituted their practical purposes or their fancy for
nature's criterion of adaptive fitness, preserving those that they wish to
perfect and eliminating those unfitted to their requirements or ideas.

In the case of many of these and other examples, wild forms still occur
which seem to be like the ancestral stock from which the domesticated
forms have been produced. All the varied forms of dogs--from mastiff to
toy-terrier, and from greyhound to dachshund and bulldog--find their
prototypes in wild carnivora like the wolf and jackal. In Asia and
Malaysia the jungle fowl still lives, while its domesticated descendants
have altered under human direction to become the diverse strains of the
barnyard, and even the peculiar Japanese product with tail feathers
sometimes as long as twenty feet. That far-reaching changes can be brought
about in a relatively short time is proved by the history of the game
cock, which has nearly doubled in height since 1850, while at the same
time its slender legs, long spurs, and other qualities have been perfected
for the cruel sport for which it has been bred. Again, the wild rock
pigeon seems to be the ancestral form from which the fantail and pouter
and carrier-pigeon with their diverse characters have taken their origin.

It is true that some biologists have urged certain technical objections to
the employment of domesticated animals and their history as analogies to
the processes and results in wild nature. To my mind, however, artificial
selection is truly a part of the whole process of natural selection. Man
is but one element of the environment of tame forms, and his fancy or need
is therefore one of the varied series of external criteria that must be
met if survival is to be the result; failing this, elimination follows as
surely as under the conditions of an area uninhabited or uninfluenced by
mankind. Congenital variation is real, selection is real and the heredity
of the more fit modification is equally real. Surely Darwin was right in
contending that the facts of this class amplify the conception of natural
selection developed on the basis of an analysis of wild life.

* * * * *

Knowing the elements of the selective process, it is possible to analyze
and to understand many significant phenomena of nature, and to gain a
clearer conception of the results of the struggle for existence,
especially when the human factor is involved. Let us see how much is
revealed when the foregoing results are employed in a further study of
some of nature's vital situations.

As a consequence of the many-sided struggle for existence, the
interrelations of a series of species will approach a condition of
equilibrium in an area where the natural circumstances remain relatively
undisturbed for a long time. For example, among the field-mice of one
generation, just as many individuals will survive as will be able to find
food and to escape hereditary foes such as cats and snakes and owls. The
number of owls, in their turn, will be determined by the number of
available mice and other food organisms, as well as by the severity of the
adverse circumstances that cause elimination of the less fit among the
fledglings brought into the world. The vital chain of connections is
sometimes astonishingly long and intricate. One remarkable illustration is
given by Fiske, as an elaboration of an example cited by Darwin. He points
out that the fine quality of the traditional roast beef of England is
directly determined by the number of elderly spinsters in that country.
The chain of circumstances is as follows: the quality of the clover
fields, furnishing the best food for cattle, depends largely upon the
visits to the clover-blossoms by wild bees, that accomplish the
fertilization of the flowers by carrying pollen upon their bodies from one
plant to another. Field-mice devour the young in the nests of these bees,
so if there are few field-mice there will be many bees, and consequently
better grazing for the cattle. The number of field-mice will vary
according to the abundance of cats, and so the number of these domestic
animals will exert an influence upon the whole foregoing chain of forms.
But, as Fiske points out, cats are the favorite companions of elderly
spinsters; therefore, if there are many of the latter, there will be more
cats, fewer field-mice, more bees, richer clover fields, and finer cattle!
Each link is real and the whole chain is a characteristic example of the
countless ways that the natural destinies of living things are
interrelated and intertwined.

The reality of such organic interrelationships is revealed with wonderful
clearness in the numerous instances where some disturbing factor has
altered one or another element of the balanced system. The invasion of the
new world by Europeans has directly led to the partial or complete
extinction of the tribes of Indians to whom the land formerly belonged;
they have disappeared almost entirely from our state of New York, together
with the bear and wolf and many other species of animals that formerly
existed here. Wild horses and bison have also vanished before the advances
of civilization and the alteration of their homes. Sometimes the
extermination of one pest has resulted in an increase in the number of
another through human interference with nature's equilibrium. In some of
our Western states, a bounty was offered for the scalps of wolves, so as
to lessen the number of these predatory foes of sheep. But when the wolves
were diminished in number, their wild food-animals, the prairie dogs,
found their lot much bettered, and they have multiplied so rapidly that in
some places they have become even more destructive than the wolves.

One of the most remarkable illustrations is that of the rabbits introduced
into Australia. This island continent was cut off from the surrounding
lands long before the higher mammals evolved in far distant regions, so
that the balance of nature was worked out without reference to animals
like the rabbit. When the first of these were introduced they found a
territory without natural enemies where everything was favorable. They
promptly multiplied so rapidly that within a few years their descendants
were numerous enough to eat up practically every green thing they could
reach. Two decades ago, the single province of Queensland was forced to
expend $85,000,000 in a vain effort to put down the rabbit plague. The
remarkable statement has been made that in some places nature has taken a
hand in causing a new type of rabbit to evolve. Finding the situation
desperate, some of the animals have begun to develop into tree-climbing
creatures. The animals exist in such numbers that the available food upon
the ground is insufficient for all, and so some elimination results. But
the young rabbits with longer claws, varying in this way on account of
congenital factors, have an advantage over their fellows because they can
climb some of the trees and so obtain food inaccessible to the others. If
the facts are correctly reported, and if the process of selection on the
basis of longer claws and the climbing habit is continued, the original
type of animal is splitting up into a form that will remain the same and
live upon the ground, and another that will be to all intents and purposes
a counterpart of our familiar squirrel. All the evidence goes to show that
squirrels have evolved from terrestrial rodents; if the data relating to
Australian rabbits are correct, nature is again producing a squirrel-like
animal by evolution in a region where the former natural situation has
been interfered with by man.

The laws of biological inheritance have received close and deep study by
numerous investigators of Darwinian and post-Darwinian times, because from
the first it was clearly recognized that a complete description of
nature's method of accomplishing evolution must show how species maintain
the same general characteristics from generation to generation, and also
how new qualities may be fixed in heredity as species transform in the
course of time. Before our modern era in biology, the fact of inheritance
was accepted as self-sufficient; now much is known that supplements and
extends the incomplete account given by natural selection of the way
evolution takes place.

It is not possible in the present brief outline to describe all the
results of recent investigations, but some of them are too important to be
passed over. Perhaps the most interesting one is that the laws of heredity
seem to be the same for man and other kinds of living creatures, as proved
by Galton and Pearson and many others who have dealt with such characters
as human stature, human eye color, and an extensive series of the
peculiarities of lower animals and even of plants.

The researches dealing with the physical basis of inheritance and its
location in the organism have yielded the most striking and brilliant
results. Darwin himself realized that the doctrine of natural selection
was incomplete, as it accepted at its face value the inheritance of
congenital racial qualities without attempting to describe the way an egg
or any other germ bears them, and he endeavored to round out his doctrine
of selection by adding the theory of pangenesis. According to this, every
cell of every tissue and organ of the body produces minute particles
called gemmules, which partake of the characters of the cells that produce
them. The gemmules were supposed to be transported throughout the entire
body, and to congregate in the germ-cells, which in a sense would be
minute editions of the body which bears them, and would then be capable of
producing the same kind of a body. If true, this view would lead to the
acceptance of Lamarck's or even Buffon's doctrine, for changes induced in
any organ by other than congenital factors could be impressed upon the
germ-cell, and would then be transported together with the original
specific characters to future generations. Darwin was indeed a good
Lamarckian.

But the researches of post-Darwinians, and especially those of the
students of cellular phenomena, have demonstrated that such a view has no
real basis in fact. Many naturalists, like Naegeli and Wiesner, were
convinced that there was a specific substance concerned with hereditary
qualities as in a larger way protoplasm is the physical basis of life. It
remained for Weismann to identify this theoretical substance with a
specific part of the cell, namely, the deeply staining substance, or
chromatin, contained in the nucleus of every cell. Bringing together the
accumulating observations of the numerous cytologists of his time, and
utilizing them for the development of his somewhat speculative theories,
Weismann published in 1882 a volume called "The Germ Plasm," which is an
immortal foundation for all later work on inheritance. The essential
principles of the germ-plasm theory are somewhat as follows. The chromatin
of the nucleus contains the determinants of hereditary qualities. In
reproduction, the male sex-cell, which is scarcely more than a minute mass
of chromatin provided with a thin coat of protoplasm and a motile organ,
fuses with the egg, and the nuclei of the two cells unite to form a double
body, which contains equal contributions of chromatin from the two
parental organisms. This gives the physical basis for paternal inheritance
as well as for maternal inheritance, and it shows why they may be of the
same or equivalent degree. When, now, the egg divides, at the first and
later cleavages, the chromatin masses or chromosomes contained in the
double nucleus are split lengthwise and the twin portions separate to go
into the nuclei of the daughter-cells. As the same process seems to hold
for all the later divisions of the cleavage-cells whose products are
destined to be the various tissue elements of the adult body, it follows
that all tissue-cells would contain chromatin determinants derived equally
from the male and female parents. As of course only the germ-cells of an
adult organism pass on to form later generations, and as their content of
chromatin is derived not from the sister organs of the body, but from the
original fertilized egg, there is a direct stream of the germ plasm which
flows continuously from the germ-cell to germ-cell through succeeding
generations. It would seem, therefore, that the various organic systems
are, so to speak, sister products in embryonic origin. The reproductive
organs are not produced by the other parts of the body, but their cells
are the direct descendants of the common starting-point namely, the egg.
As the cells of the reproductive organs are the only ones that pass over
and into the next and later generations, it will be evident, in the first
place, that the germ plasm of their nuclei is the only essential substance
that connects parent and offspring. This stream of germ plasm passes on in
direct continuity through successive generations--from egg to the complete
adult, including its own germ-cells, through these to the next adult, with
its germ-cells, and so on and on as long as the species exists. It does
not flow circuitously from egg to adult and then to new germ-cells, but it
is direct and continuous, and apparently it cannot pick up any of the
body-changes of an acquired nature. Now we see why individual acquisitions
are not transmitted. The hereditary stream of germ plasm is already
constituted before an animal uses its parts in adult life; we cannot see
how alterations in the structure of mature body parts through use and
adjustment to the environment can be introduced into it to become new
qualities of the species.

It must be clear, I am sure, that this theory supplements natural
selection, for it describes the physical basis of inheritance, it
demonstrates the efficiency of congenital or germ-plasmal factors of
variation in contrast with the Lamarckian factors, and finally in the way
that in the view of Weismann it accounts for the origin of variations as
the result of the commingling of two differing parental streams of germ
plasm.

At first, for many reasons, Weismann's theories did not meet with general
acceptance, but during recent years there has been a marked return to many
of his positions, mainly as the result of further cytological discoveries,
and of the formulation of Mendel's Law and of De Vries's mutation theory.
The first-named law was propounded by Gregor Mendel on the basis of
extensive experiments upon plants conducted during many years, 1860 and
later, in the obscurity of his monastery garden at Altbruenn, in Austria.
It was rescued from oblivion by De Vries, who found it buried in a mass of
literature and brought it to light when he published his renowned Mutation
Theory in 1901. Mendelian phenomena of inheritance, confirmed and extended
by numerous workers with plants and animals, prove that in many cases
portions of the streams of germ plasm that combine to form the hereditary
content of organisms may retain their individuality during embryonic and
later development, and that they may emerge in their original purity when
the germ-cells destined to form a later generation undergo the preparatory
processes of maturation. They demonstrate also the apparent chance nature
of the phenomena of inheritance. To my mind the most striking and
significant result in this field is the demonstration that a particular
chromosome or chromatin mass determines a particular character of an adult
organism, which is quite a different matter from the reference of all the
hereditary characters to the chromatin as a whole. Wilson and others have
brought forward convincing proof that the complex character of sex in
insects actually resides in or is determined by particular and definite
masses of this wonderful physical basis of inheritance.

Mendel's principles also account in the most remarkable way for many
previously obscure phenomena, like reversion, or a case where a child
resembles its grandparent more than it does either of its parents; such
phenomena are due, so to speak, to the rise to the surface of a hidden
stream of germ plasm that had flowed for one or many generations beneath
its accompanying currents. I believe that the law is replacing more and
more the laws of Galton and Pearson, formulated as statistical summaries
of certain phenomena of human inheritance taken _en masse_. According to
Galton's celebrated law of ancestral inheritance, the qualities of any
organism are determined to the extent of a certain fraction by its two
parents taken together as a "mid-parent," that a smaller definite fraction
is contributed by the grandparents taken together as a mid-grandparent,
and so on to earlier generations. But Mendel's Law has far greater
definiteness, it explains more accurately the cases of alternative
inheritance, and it may be shown to hold for blended and mosaic
inheritance as well.

De Vries's new "mutation theory" is clearly not an alternative but a
complementary theory to natural selection, the Weismannian and Mendelian
theories. Like these last, it emphasizes the importance of the congenital
hereditary qualities contained in the germ plasm, though unlike the
Darwinian doctrine it shows that sometimes new forms may arise by sudden
leaps and not necessarily by the slow and gradual accumulation of slight
modifications or fluctuations. The mutants like any other variants must
present themselves before the jury of environmental circumstances, which
passes judgment upon their condition of adaptation, and they, too, must
abide by the verdict that means life or death.

From what has been said of these post-Darwinian discoveries, the
Lamarckian doctrine, which teaches that acquired non-congenital characters
are transmitted, seems to be ruled out. I would not lead you to believe
that the matter is settled. I would say only that the non-transmission of
racial mutilations, negative breeding experiments upon mutilated rats and
mice, the results of further study of supposedly transmitted immunity to
poisons--that all these have led zooelogists to render the verdict of "not
proved." The future may bring to light positive evidence, and cases like
Brown-Sequard's guinea-pigs, and results like those of MacDougal with
plants, and of Tower with beetles, may lead us to alter the opinion
stated. But as it stands now most investigators hold that there are strong
general grounds for disbelief in the principle, and also that it lacks
experimental proof.

* * * * *

The explanation of natural evolution given by Darwinism and the principles
of Weismann, Mendel, and De Vries, still fails to solve the mystery
completely, and appeal has been made to other agencies, even to teleology
and to "unknown" and "unknowable" causes as well as to circumstantial
factors. A combination of Lamarckian and Darwinian factors has been
proposed by Osborn, Baldwin, and Lloyd Morgan, in the theory of organic
selection. The theory of orthogenesis propounded by Naegeli and Eimer, now
gaining much ground, holds that evolution takes place in direct lines of
progressive modification, and is not the result of apparent chance. Of
these and similar theories, all we can say is that if they are true, they
are not so well substantiated as the ones we have reviewed at greater
length.

The task of experimental zooelogy is to work more extensively and deeply
upon inheritance and variation, combining the methods and results of
cellular biology, biometrics, and experimental breeding. We may safely
predict that great advances will be made during the next few years in
analyzing the method of evolution; and that a few decades hence men will
look back to the present time as a period of transition like the era of
reawakened interest and renewed investigation that followed the appearance
of the "Origin of Species." For the present, we can justly say "that
evolution, so far as it is understood, is a real and natural process."

V

THE PHYSICAL EVOLUTION OF THE HUMAN SPECIES AND OF HUMAN RACES

The teachings of science that relate to the origin and history of the
human species constitute for us the most important part of the whole
doctrine of organic evolution and now, having completely outlined this
doctrine as a general one, we are brought to the point where we must deal
frankly and squarely with the insistent questions arising on all sides as
to the way that mankind is involved in the vast mechanism of nature's
order. These questions have been ignored heretofore, in order that the
natural history of animals in general might be discussed without any
interference on the part of purely human interest and concern. It now
becomes our privilege, and our duty as well, to employ and apply the
principles we have learned in order to understand more completely the
origin of the human body as an organic type, the history of human races,
the development of human faculty and of social institutions, and the
evolution finally of even the highest elements of human life. These are
scientific problems, and if we are to solve them we must employ the now
familiar methods of science which only yield sure results.

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