THE POST-DARWINIANS: DEVRIES AND THE
MUTATION THEORY.
The period between 1860 and 1900 was occupied largely by elaborations
of the Darwinian conception of evolution, and arguments as to whether
or not organic descent was a fact. In those four decades there were
many famous workers--Alfred Russell Wallace, co-discoverer with
Darwin of the theory of selection; Weismann and Haeckel, Germany’s
great evolutionists; the philosopher, Spencer; Cope, the American
paleontologist, and Huxley, the English champion of scientific
rationalism--these, and a host of others spent their lives in
demonstrating the workings of evolution. But unfortunately, the
opposition which they encountered forced them to write and work
largely along lines of argument and thus much of their work was
fruitless so far as the discovery of new principles is concerned.
During this same period the doctrine of evolution suffered much
from over-enthusiasm on the part of some of its defenders. Even
Wallace overdid the hypothesis of sexual selection, and the kindred
hypotheses of concealing and protective coloration. Naturalists
sought to explain every coloring of animals and plants as being of
some value to them, and therefore the real cause of the existence
of the species; not a few carried the idea of value in sexual
differences, such as those between the male and female peacock,
to a similar extreme. But in spite of the inaccuracies which they
published, these enthusiasts did far more good than harm, for they
aided greatly in securing popular support for the main theory.
It was toward the beginning of this century that evolutionary
studies received another great stimulus. Professor Hugo de Vries,
a Dutch botanist of considerable note, proposed what he called the
“mutation theory” as a substitute for Darwin’s conception of “natural
selection.” He began his studies by attempting to produce by careful
selection a variety of buttercup which should contain in its flower
more than the normal number of petals. He actually achieved the
desired increase, but it was far from a stable condition; while
some of the flowers possessed eight, nine, or ten petals, and a few
as high as thirty-one, many of them possessed the original number,
five. When selection was abandoned there appeared at once a general
retrogression toward the primitive state, and this fact caused de
Vries to conclude that selection alone was not enough to cause
the formation of a new species of plant or animal[17]. Instead,
he concluded that when a change of permanent value took place in
a plant or animal it was something entirely different from the
constant variations on which Darwin and his followers relied; it was
a discontinuous variation--a ‘sport,’ the florist or gardener would
call it--to which de Vries applied the new name mutation. Mutation,
he believed, involved a very definite change in the reproductive
cells of the organism--a change which had absolutely no relation to
the environment. They arose from conditions within the plant and
animal, and might or might not affect it favorably. Those mutations
which were not beneficial would be eliminated by selection; those
which were of value to the creature would probably be preserved.
Thus, in de Vries’ mind evolution was a process due primarily to
internal causes, its course being merely guided by environment,
which selected those mutations capable of surviving.
Without question, de Vries had a real basis for his theory. Mutations
do take place among both wild and domestic creatures; thus among
the dandelions there constantly appear special types which breed
true and are, as Castle has called them, “little species within the
dandelion species.” Similar mutations are well known in peas, beans,
evening primroses, and such domestic animals as the sheep. Clearly,
therefore, species do arise as de Vries stated; the question is, is
this the only way in which they arise?
This problem was raised little more than twenty years ago--a period
far too short to allow for the settling of a question that is merely
another statement of the problem that has puzzled scientists and
philosophers for more than twenty centuries.
There is, however, excellent reason for believing that the
conceptions of both de Vries and Darwin are true; that neither of
them excludes the other from operation. Thus in the famous chalk
formation of England there may be found an evolutionary chain of
sea urchins which, according to the general consensus of opinion,
represent true Darwinian evolution. As N. C. Macnamara says, “They
are first found in their shelled, sparsely ornamented forms, from
which spring, as we ascend the zone, all the other species of the
genus. The progression is unbroken and minute in the last degree. We
can connect together into continuous series each minute variation
and each species of graduation of structure so insensible that not a
link in the chain of evidence is wanting.”
On the other hand, the writer has recently completed a microscopic
study of a group of ancient lamp-shells--animals which looked somewhat
like molluscs, but which were very different internally--with
altogether different results. The particular changes involved were
minor matters of surface markings, which could have had no conceivable
importance to the animals. Selection, therefore, may be virtually
ruled out; indeed, many of the different forms lived close together,
with apparently equal success. But in the small markings on the shells
there appear, as one follows the series from bottom to top, very
decided changes, and those changes are, in some cases, abrupt and
complete.
In others the variations are very small--indeed they could be
distinguished only with the microscope--but so far as could be told,
were distinct. This, therefore, points to a course of evolution that
was clearly a matter of mutation, without any apparent governing by
the process of natural selection.
The conclusion which we may reach, therefore, is that both natural
selection and mutation operate in the development of new forms
from old. The variations, for which Darwin was at a complete loss
to account, are in many cases the mutations emphasized by de Vries
and his followers. But to what extent climate, food, habits, and
multitudinous other environmental factors, coupled with such
internal ones as racial old age, complicate the processes of
variation and selection cannot yet be said. De Vries, in his mutation
theory, supplied one of the deficiencies of Darwinism, and at the
same time led scientists in general to realize that evolution is a
far more complex problem than was supposed during the later portion
of the last century. Darwin’s primitive mudfish, with its trace of
mind, and the process of natural selection, will not by any means
account for the multitude of higher vertebrate forms which people,
and have peopled the lands and waters of the globe.
At the same time the scientific public was awaking to the fact that
evolution was an almost inconceivably complex affair, many of the
post-Darwinian hypotheses began to show themselves of very doubtful
importance. The theory of sexual selection, which Darwin elaborated
in the “Descent of Man” began a steady decline. Such selection
undoubtedly does take place, but it is not carried on to so great
an extent as was once supposed. The idea of the protective value of
colors and color arrangement, too, began to be doubted, although at
the same time its principles became much better known and therefore
more strongly emphasized by some naturalists. Inheritance of directly
acquired characters was proved to be an impossibility, and much
doubt was thrown upon the hypothesis of use and disuse. Instead of
legs disappearing because they are not used, they are now thought
to disappear because the evolutionary processes going on within
the animal demands their disappearance. What these processes are we
do not know, but our frank avowal of ignorance gives us a certain
confidence that we shall eventually find out.
But it is not only ideas that have changed within the last two
decades; methods of study have undergone an even greater revolution.
De Vries, at almost the same time he discovered mutation,
rediscovered the fact that heredity was by no means so mysterious
and erratic as it had been generally thought. Animals and plants, he
discovered, possessed many characters which behaved in very definite
ways when two varieties were crossed, and that the characters of an
organism could be determined largely by the interbreeding of its
ancestors. Thus arose the science of _genetics_, which seeks to
find out the numerous factors underlying the various phenomena of
heredity. And since heredity is the base of all evolution, genetics
has for its ultimate aim the determination of the causes of that
great process which is responsible for the existence of whatever
animals and plants inhabit and have inhabited the earth. The
geneticist is the most modern of evolutionists; he is not satisfied
with finding out what has taken place in the past; he sets out to
make evolution, or tiny portions of it, take place within his own
laboratories and greenhouses.
* * * * *
Today, despite the assertions of a few of its opponents, the theory
of organic evolution is more thoroughly alive than it has ever
been before. Paleontologists are studying their fossil shells and
corals and bones in order to find out what has taken place during
the millions upon millions of years during which living things
have inhabited our planet. Anatomists are studying the bodies of
modern animals, from the simplest to the highest, to determine their
relationships one to the other; embryologists are tracing out the
evolution of the individual in his life before birth. The geneticists
are breeding plants, rabbits, mice, fishes, flies, potato bugs so
that they may discover what evolution is doing today. Everywhere
men are studying, comparing, experimenting. Their purpose is not
to discover whether or not evolution is a fact; on that point they
have long ago been satisfied. They are trying to find out how it
operates and what forms it has produced; how differences arise among
organisms, and what are their effects, and by what means they are
passed from one generation to another until they become part and
parcel of the inheritance, thereby establishing a new species.
FOOTNOTES:
[1] Modified after Zeller and Osborn.
[2] Osborn, “From the Greeks to Darwin,” p. 86.
[3] This claim has at various times been disputed;
Osborn, however, accepts it without question.
[4] “From the Greeks to Darwin,” pp. 101-102.
[5] Quoted by Osborn, with the comment: “As Haeckel
observes, Darwin rose up as Kant’s Newton.”
[6] Osborn, on whose writings most of this chapter is
based, comments that Scotland was “a country which
the Mayor evidently considered so remote that
his observation would probably not be gainsaid.”
This important fact, that the faker could not be
contradicted, probably was responsible for many of
the absurdities published. However, when we examine
the general state of knowledge at that time, we
are forced to admit that this is not the whole
explanation. Without much question, many of these
writers were at least partly serious, and actually
believed the impossible tales which they printed,
just as they believed they had seen witches and
ghosts.
[7] The “Scientific Monthly” contains an interesting
article on the history of scientific illustration,
showing many of the remarkable pictures to be found
in early works.
[8] Carl von Linne was the greatest naturalist of
eighteenth century Sweden. He lived from 1707 to
1778, and for many years was professor at the
University of Upsala.
[9] In Buffon’s day the Americas were still the “New
World,” and it was customary with naturalists of the
time to consider it new, not only in discovery, but
in its plant and animal inhabitants. For them, the
animals of America came from the Old World, just
as did its white settlers; the idea of opposite
migrations was quite unheard of. How different this
conception was from the actual state of affairs can
be seen by reference to such books as Osborn’s “Age
of Mammals.”
[10] Osborn, op. cit. p. 138.
[11] Op. cit., pp. 181-182. The need of which Dr. Osborn
speaks was not by any means confined to science of
Goethe’s time. The great characteristic of modern
paleontology, for example, is observation without
either generalization or philosophy. It is for this
reason that the science of fossils has yielded
relatively meagre data on evolution.
[12] This was not true of the naturalist in later life,
when he was for years a semi-invalid.
[13] “Contemporary Portraits,” pp. 12-13.
[14] “Mammifers” = mammals; that is, animals which suckle
their young.
[15] Darwin seemed unable to speak of Lamarck without
contempt or derision. Certainly he was not familiar
with Lamarck’s writings in the French, and
attributed to that naturalist certain erroneous
ideas for which he was not responsible. Also, it
would seem that Darwin failed to make allowances for
Lamarck’s insuperable handicaps, and his position
as a pioneer, and therefore adopted an attitude of
unjustified antagonism.
[16] “Darwinism,” or “The Darwinian Theory” refers to the
theory of natural selection, and the sub-theory of
sexual selection, ~not~ to the theory or concept of
organic evolution.
[17] This conclusion was probably unjustified; his
observation covered too short a period to mean a
great deal.