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of Lamarck, Geffroy-Saint-Hilaire, and more especially the glorious
name of Charles Darwin.

According to these theories, the environment is regarded as the chief
cause of the evolution of organic forms. Charles Darwin, who formulated
the best and most detailed theory of evolution, based it on the two
principles of the _variability_ of living organisms, and _heredity_,
which transmits their characteristics from generation to generation.
And in explanation of the underlying cause of evolution, he expounded
the doctrines of the _struggle for existence_ and the _natural
selection_ of such organic forms as succeeded to a sufficient degree in
adapting themselves to their environment.

Whatever the explanation may be, the substantial fact remains of the
_variability of species_ and the successive and gradual transition
from lower to higher forms. In this way, the higher animals and plants
must have had as antecedents other forms of _inferior species_, of
which they still bear more or less evident traces; and in applying
these theories to the interpretation of the personalities of human
degenerates, he frequently invoked the so-called principle of
_atavism_, in order to explain the reappearance of atavistic traits
that have been outgrown in the normal human being, certain anomalies
of form more or less analogous to parallel forms in lower species of
animals.

There are other theories of evolution less familiar than that of
Darwin. Naegeli, for instance, attributes the variability of species
to _internal_, rather than external causes - namely, to a spontaneous
activity, implanted in life itself, and analogous to that which is
witnessed in the development of an individual organism, from the
primitive cell up to the final complete development; without, however,
attributing to the progressive alterations in species that predestined
final goal which heredity determines in the development of individual
organisms.

The internal factor, namely life, is the primary cause of _progress_
and the _perfectionment_ of living creatures - while environment
assumes a secondary importance, such as that of _directing_ evolution,
acting at one time as a stimulus toward certain determined directions
of development; at another, permanently establishing certain useful
characteristics; and still again, effacing such forms as are unfit.

In this way the external causes are associated with evolution, but
with very different effects from those attributed to them by Darwin,
who endowed them with the creative power to produce new organs and new
forms of life.

Naegeli compared the internal forces to invested capital; it will draw
a higher or lower rate of interest, according as its environment proves
to be more or less favourable to earning a profit.

The most modern theory of evolution is that of De Vries, who, after
having witnessed the spontaneous and unforeseen transformations of a
certain plant, the OEnohtera Lamarckiana, without the intervention of
any external phenomenon, admitted the possibility of the unexpected
occurrence of other new forms, from a preexistent parent form - and to
such phenomena he gave the name of _mutations_.

It is these _mutations_ that create new species; the latter, although
apparently unheralded, were already _latent_ in the germ before they
definitely burst into life. Consequently, new species are formed
potentially in the germinating cells, through spontaneous activity.

The characteristics established by _mutations_ are hereditary, and the
species which result from them persist, provided their environment
affords favourable conditions, better suited to them than to the
preexisting parent form.

Accordingly new species are _created_ unexpectedly. De Vries draws a
distinction between mutations and variations, holding that the latter
are dependent upon environment, and that in any case they constitute
simple _oscillations of form_ around the normal type determined in each
species by mutation.

Species, therefore, cannot be transformed by external causes or
environments, and the mechanism of transformation is not that of a
succession of very gradual variations, which have given rise to the
familiar saying: _natura non facit saltus_. On the contrary, what
produces stable characteristics is a _revolution_ prepared in a latent
state, but unannounced in its final disclosure. A parallel to this is
to be found, for example, in the phenomena of _puberty_ in its relation
to the evolution of the individual.

Now, when a species has once reached a fixed stability as regards its
characteristics, it is _immutable_, after the analogy of an individual
organism that has completed its development; henceforth its further
evolution is ended. In such a case, the oscillations of variability are
exceedingly limited, and adaptation to new environments is difficult;
and while a species may offer the appearance of great strength (_e.g._,
certain species of gigantic extinct animals), it runs the risk of dying
out, because of a lower potentiality of adaptability; or, according to
the theory of Rosa, it may even become extinct spontaneously.

Accordingly it is not the fixed species that continue the process
of _evolution_. If we compare the tree of life to a plant, we may
imagine evolution as soaring upward, sustained by roots far below; the
new branches are not put forth by the old branches, but draw their
sustenance from the original sources, from which the whole tree draws
its life. When a branch matures and flowers, it may survive or it may
wither but it cannot extend the growth of the tree.

Furthermore, the new branches are always higher up than the old ones;
that which comes last is the highest of all.

Thus, the species which are the _latest_ in acquiring a stable form
are the highest up in the biological scale, because the privilege of
carrying forward the process of evolution belongs to those species
which have not yet become fixed. An apparent weakness, instability,
an active capacity for adaptation, are consequently so many signs of
_superiority_, as regards a potential power of evolution - just as the
nudity and sensibility of animal cells, for example, are signs of
superiority, as compared with vegetable cells - and of man, as compared
with the lower animals.

In order to show that the inferiority of a species is in proportion to
its precocity in attaining fixed characteristics, Rosa conceived the
following striking comparison. Two animals are fleeing, along the same
road, before an advancing flood. One of the two climbs to the top of a
neighboring tree, the other continues in its flight toward a mountain.
As the level of the water rises, it threatens to isolate and engulf
the animal now stalled upon the tree; the other animal, still fleeing
toward the heights, reaches, on the contrary, a higher and more secure
position.

The animal on the tree stands for an inferior species that has earlier
attained a fixed form; the other represents a higher species that
has continued to evolve; but the animal upon the mountain never was
on the tree at all, because, if he had mounted it and become caught
there, he would have lost his chance of continuing on his way. In other
words, the _higher_ species never was the _lower_ species, since the
characteristics of the latter are already fixed.

Some eloquent comparisons might be drawn from the social life of
to-day. We are all of us spurred on to choose as early as possible some
form of employment that will place us in a secure and definite place
at the great banquet of existence. The idea of continuing to follow an
indefinite and uncertain path, leading upward toward the heights is far
less attractive than the safe and comfortable shelter of the shady tree
that rises by the wayside. The same law of inertia applies to every
form of life. Biological evolution bears witness to it, in the _forms_
of the different species; social evolution, in the _forms_ of the
professions and trades; the evolution of thought, in the _forms_ of the
different faiths. And whoever first halts in any path of life, the path
of study, for instance, occupies a lower place than he who continues on
his road.

The salaried clerk, armed only with his high-school certificate, has
an assured income and the pleasures of family life, at a time when
the physician, with an independent profession, is still struggling to
establish a practice. But the obscure clerk will eventually hold a
social position below that of the physician; his income will always be
limited, while the physician may acquire a fortune. Now, the clerk,
by _adapting_ himself to his bureaucratic environment, has acquired
certain well-defined characteristics; we might even say that he has
become a representative type of the _species_, clerk. And the same
will be true of the physician in his independent and brilliant life as
high priest of humanity, scientist and man of wealth. Both men were
high-school students, and now they are two widely different social
types; but the physician never represented the type of clerk; or,
in other words, he did not have to be a clerk before he could be a
physician; on the contrary, if he had been a clerk, he never could have
become a physician. It is somewhat after this fashion that we must
conceive of the sequence of species in evolution. It follows that man
never was an anthropoid ape, nor any other animal now living around
us. Nor was the man of the white race ever at any time a negroid or
a mongolian. Consequently, the theory is untenable which tries to
explain certain morphological or psychic malformations of man, on
the principle of atavism - because no one can inherit if he is not a
descendant.

So, for example, reverting to our previous comparisons, if the animal
on the mountain should climb a tree, or if the physician should become
pedantic, this would not prove that the animal from the mountain
was once upon a time the animal in the tree, nor that the physician
recalled, by his eventual pedantry, certain bygone days when he was a
clerk.

The theories of evolution seemed for a time to illumine and definitely
indicate the origin of man. But this illusion has so far resulted only
in relegating to still deeper darkness the truth that the biologists
are seeking. We do not know of whom man is the son.

Even the earlier conceptions regarding the mechanics of evolution are
essentially altered. The mystery of the origin of species, like that of
the mutability of forms, has withdrawn from the forms that are already
developed, and taken refuge in the _germinal cells_; these cells in
which no differentiation is revealed, yet in which the future organism,
in all its details, exists in a potential state; in which, we may even
say, _life exists independent of matter_, are the real _laboratorium
vitæ_. The individual, in developing, does nothing more than _obey_, by
fulfilling the potentiality of the germs.

The direction of research has shifted from the individual to its germs.
And just as the early Darwinian theories evolved a _social ethics_,
seemingly based upon the facts of life, to serve as a guide in the
_struggle for existence_, so in the same way, to-day, there has arisen
from the modern theories a new _sexual ethics_, founded upon a biologic
basis.

=The Phenomena of Heredity.= - The most interesting biological
researches of to-day are in regard to the hereditary transmission of
characteristics.

To-day the phenomena of heredity are no longer absolutely obscure,
thanks to the studies of Mendel, who discovered some of its laws,
which seemed to open up new lines of research prolific in results.
Yet even now, although this field has been invaded by the most
illustrious biologists of our time, among others, De Vries, Correns,
Tschermack, Hurst, Russell, it is still in the state of investigation.
Nevertheless, the _general trend_ of researches relative to Mendel's
laws is too important to permit of their enlightening first steps being
neglected by Anthropology.

The first phenomena observed by Mendel, and the ones which led him
to the discovery of the laws of heredity which bear his name, were
revealed by a series of experiments conducted with peas.

_Exposition of the Phenomena of Hybridism._ - If two strains of peas are
crossed, one of them having red flowers and the other white flowers,
the result in the first generation is, that all the plants will have
red flowers, precisely similar to those of one of the parent plants.

Accordingly, in hybridism, the characteristic of one of the parents
completely hides that which is antagonistic to it in the other parent.
We call this characteristic (in the case cited, the red flowers),
_dominant_; in distinction to the other characteristic which is
antagonistic to the first and overcome by it; namely, the recessive
characteristic (in the present case, the white flowers). This is the
law of prevalence, and constitutes Mendel's first law, which is stated
as follows:

_Mendel's First Law_: "When antagonistic varieties or characteristics
are crossed with each other, the products of the first generation are
all uniform and equal to one of the two parents."

This result has been repeatedly reached in a host of researches, which
have experimentally established this phenomenon _as a law_.

Thus, for example, if we cross a nettle having leaves with an indented
margin, with a nettle having leaves with a smooth margin, the product
of the first generation will all have leaves with indented margins, and
apparently identical with the parent plant having indented margins,
in other words, having the characteristic that has proved itself the
dominant one (Russell).

These phenomena discovered by Mendel have been observed in many
different species of plants, such as wheat, Indian corn, barley and
beans.

They have also been verified in certain animals, such as mice, rats,
rabbits, caveys, poultry, snails, silk-worms, etc. One of the most
typical experiments was that of Cuénot, who, by crossing ordinary mice
with jumping mice, obtained as a result a first generation composed
wholly of normal mice; the characteristic of jumping was thus shown to
be recessive.

Notwithstanding that the first generation is apparently in every way
similar to the parent with the dominant character, there is in reality
a difference.

Because, if we cross these hybrids _together_, we meet, in the second
generation, with the following phenomenon: to every three individuals
possessing the dominant character, one is born having the recessive
character. To go back to Mendel's first example, that of the peas with
red flowers (dominant) and with white flowers (recessive), we find, by
crossing together the hybrids of the first generation, that for every
three plants with red flowers, there is one plant with white flowers.

And similarly, the crossing of hybrid nettles with indented leaves will
result in a second generation composed of three plants with indented
leaves to every one with smooth-edged leaves (see Fig. 5).

[Illustration: FIG. 5.]

That is, the characteristics which belonged to the first two parents
all survive, even though in a latent form, in the descendants; and they
continue to differentiate themselves in well established proportions.
In one offspring out of four, the characteristics of the grandfather,
which have remained dormant in the father, once more reappear. This
intermittent heredity of characteristics, that are passed from
grandfather to grandson, overleaping the father, is one of the
best-known laws of _pathological_ _heredity_ in man; and it is called
_atavistic heredity_, to distinguish it from _direct heredity_, which
denotes the transmission from parent to offspring. But no explanation
had ever been found for this sort of phenomenon. Undoubtedly, it must
be connected with the phenomena of Mendelism.

Accordingly, in the second generation Mendel's second law has been
established, the _law of disjunction_, which is stated as follows:

_Mendel's Second Law_: "In the second generation obtained by reciprocal
fertilisation of the first hybrids, three quarters of the offspring
will exhibit the dominant character, and one quarter the recessive."

_Mendel's Hypothesis, Designed to Explain the Phenomena of_
_Heredity._ - Mendel's great service is to have conceived a hypothesis
that seems to have disclosed the key adapted to unlock all the secrets
of heredity.

While the body of an individual is the resultant of forces so
mutually exclusive that the appearance of one characteristic means
the disappearance of its antagonist; _in the development of_ _the
sexual cells the two antagonistic characters are distributed in equal_
_proportion_. That is to say, one-half of the male cells contain the
dominant character, and one-half the recessive; and the same holds
true for the female cells. The characters of the two parents, in other
words, never _merge_ in the reproductive cells, but are distributed _in
equal measure_, independently of the question whether they are dominant
or recessive. Thus for example: in the case already cited of the first
hybrid generation of the peas with red flowers, in every one of the
plants, without distinction, half the pollen has potentially the red
character and half has the white; and in the same way the female cells
have, half of them a red potentiality and half of them a white. Such
hybrids of the first generation, therefore, although apparently similar
to the parent with red flowers, _differ in their germinative powers_,
which are not made apparent in the individual. And the same may be said
of hybrid nettles with indented leaves, etc.

Granting Mendel's hypothesis, we have on the one hand pollen and on
the other seed ready to come together in every manner included within
the range of possible combinations; the _individual_ is, in its
characteristics, nothing else than the product of a combination which
must necessarily manifest itself in accordance with the well-known
mathematical laws of _probability_.

For instance, let us proceed to diagram the possible disposition of the
sexual cells of the hybrids of peas, all of them having red flowers. In
terms of percentage, they will give, out of every hundred, fifty red
and fifty white.

_P_ = pollen; _O_ = ova; _R_ = red, dominant; _w_ = white, recessive:

The possible number of combinations between the pollen grains and
the ova are four; namely, _RR_, _Rw_, _wR_, _ww_. But where a
dominant characteristic encounters a recessive (_Rw_, _wR_), the
recessive disappears, to make way in the individual for the dominant
characteristic alone. The definitive result is three individuals of
dominant character, to one of recessive character.

[Illustration: FIG. 6.]

Nevertheless, the hybrids of dominant character are not all equal
among themselves. Those belonging to the combination _RR_, indeed, are
_permanent_ in character and in all respects alike, and they reproduce
the original red-flower progenitor. The other red-flower hybrids,
belonging to the groups _Rw_ and _wR_ are, on the contrary, similar
to the hybrids of the first generation and contain reproductive cells
differentiated in character; such hybrids, if reciprocally fertilised,
will again give three dominant offspring to every one recessive; that
is, they will obey the law of disjunction. The hybrids belonging to
the fourth group, on the contrary, are constant, like those of the
first group, and are permanently of recessive character; and they will
reproduce the original progenitor with white flowers.

The same results may be attained with nettles with smooth and indented
leaves, and with all other types of plant and animal life that obey the
laws of Mendelism.

The figure given actually represents the third generation of nettles;
from a combination corresponding to _RR_, there result only indented
leaves, and from another combination corresponding to our _ww_ there
result only smooth-edged leaves, and from the two mixed groups there
come three offspring with indented leaves to every one with smooth
leaves.

It is possible to represent, by means of a general diagram, the
mathematical succession of characteristics in hybrids, after the
following manner; denoting the dominant character by _D_, and the
recessive by _r_.

[Illustration: First crossing of individuals with antagonistic
characters.

First generation of hybrids, all alike, and similar to the progenitor
_D_ (dominant).

Second generation: for each recessive there are three dominant: but of
these only one is permanent.

Third generation: disjunction of the hybrid groups takes place and new
permanent groups are formed.

FIG. 7.]

In each successive generation, provided the fertilisation takes place
only between uniform individuals, as indicated in the diagram, and as
may be effected by actual experiment with plants, groups identical
with the original progenitors will continue to be formed, through
successive disjunction of the hybrids; the sexual phenomenon operating
in obedience to the laws of probability.

An effective experiment, that anyone may repeat for himself, is the one
originated by Darbishire. He took two boxes, typifying respectively
the male and female organ, and placed in them black and white disks of
equal size, so distributed that each box contained fifty disks of each
colour. After mixing these disks very carefully, he proceeded to take
_at random_ one disk at a time alternately from each box; and he piled
up each pair of disks in such a manner that the black ones should be
on top and the white underneath. The result was that for every three
black disks on top of the piles there was one white disk; but of the
black groups one consisted of two black disks, while in the other
two the lower disk was white. This is simply one of the many games
dependent on the laws of probability.

Now, supposing that instead of one, there are two characteristics that
are in antagonism; in that case, we have the occurrence of double
hybridism (dihybridism).

Let us take the strains of peas already considered, but let us choose
for observation the character of their seed. One of the plants has
round seed and yellow cotyledons; and the other angular seed and green
cotyledons. These two characteristics, therefore, are both inherent in
the seed; condition of surface (rough, smooth), and colour (green, and
yellow).

After fertilisation, Mendel's first law, that of the prevalence of
the dominant character, will operate, and all the plants of the first
generation will have round seed and yellow cotyledons. Hence these
are the dominant characteristics, which we will represent by capital
letters: _R_ (round), _Y_ (yellow), to distinguish them from the
recessive characteristics, which we will designate with small letters:
_a_ (angular), and _g_ (green).

According to Mendel's hypothesis, all these hybrids with round seed and
yellow cotyledons, contain sexual cells of opposite potentialities,
numerically equal and corresponding to the antagonistic characters of
the parent plants. That is, they must have in their pollen grains and
their ovarian cells all the possible combinations of their different
potentialities.

They should produce in equal quantities:

pollen grains (_P_) with round seed and yellow cotyledons: _R Y_
" " green " _R g_
angular " yellow " _a Y_
" " green " _a g_
ovarian cells (_O_) with round " yellow " _R Y_
" " green " _R g_
angular " yellow " _a Y_
" " green " _a g_

The total number of combinations that may result is sixteen; that is,
each one of the four combinations of pollen may unite with any one of
the ovarian cells; thus constituting four groups of four. And these
groups represent the combinations (of pollen and ova) capable of
producing individuals:

_R Y - R Y = R Y | a Y - R Y = R Y_
_R Y - R g = R Y | a Y - R g = R Y_
_R Y - a Y = R Y | a Y - a Y = a Y_
_R Y - a g = R Y | a Y - a g = a Y_
_ - - - - - - - - -| - - - - - - - - -_
_R g - R Y = R Y | a g - R Y = R Y_
_R g - R g = R g | a g - R g = R g_
_R g - a Y = R Y | a g - a Y = a Y_
_R g - a g = R g | a g - a g = a g_

[Illustration: FIG. 8.]

Every time that a dominant characteristic encounters a recessive
one (_R_ with _a_ or _Y_ with _g_), it overpowers and hides it:
consequently the results of the different combinations are quite



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