Such was the general condition of theoretical biology about the
beginning of the present century. In the meantime those who were dealing
with the empyrical or experimental side of these problems were seeking
for the causes of and the rules for variation. All living things vary
from one generation to another; the question was, Why do they vary? and
do these variations really represent new characters comparable to new
species in the making? or are they, so to speak, but an elastic reaction
of the internal vital elasticity of the organism, all the while latent
and only seeking a favorable expression, to return again under other
conditions to the former type?
The effort to reduce these variations to law and system was pursued by
thousands of investigators, with varying but at all times perplexing
and disappointing results. But in the year 1900 the scientific world
awoke to the surprising fact that a patient obscure investigator had
already solved most of the puzzles of variation and heredity some
thirty-five years before. Gregor Mendel, born a peasant boy, trained as
a monk, and afterwards appointed Abbot of Brünn, had in the year 1865
published the results of his experiments in breeding, which had been
ignored or forgotten until rediscovered in 1900 by de Vries and two
others simultaneously. From this point Mendelism, as it is now called,
has steadily gained ground, until at the present time it can be said to
be the dominating conception among biologists the world over regarding
the problems of heredity.
Mendel worked chiefly with peas, crossing different varieties. In his
methods of investigation he differed from all previous investigators in
concentrating his attention upon a single pair of alternative or
contrasted characters at a time, and observing how these alternative
characters are transmitted.
Thus when he crossed a tall with a dwarf, giving attention to this pair
of contrasted characters alone, he found that all the first hybrid
generation were talls, with no dwarfs and no intermediates. Accordingly
he called the tall character _dominant_, and the dwarf character
_recessive_, and a pair of contrasted characters which act in this way
are now called _factors_ or sometimes called _unit characters_. But on
allowing these hybrids to cross-fertilize one another in the usual way,
Mendel found that in the second generation of hybrids there were
_always_ _three talls to one dwarf_ out of every four. Further
experiments proved that these dwarfs of the second hybrid generation
_always bred true_, that is, one out of four; and that one out of the
remaining talls always bred true, making another quarter of the total;
while the remaining fifty per cent. proved to be mixed tails, always
acting as did the original hybrids, splitting up in the next generation
in the same arithmetical proportion as before.
Accordingly, if we confine our study to the two contrasted characters,
tallness and dwarfness, we see that just three kinds of peas exist,
namely, dwarfs which breed true, talls which breed true, and talls which
always give the same definite proportion of talls and dwarfs among their
descendants. Innumerable experiments which have since been made with
other pairs of characters have demonstrated that this same mathematical
proportion holds good throughout the whole world of plants and
animals;[25] and hence this astonishing result is now called Mendel's
Law, and is regarded as the most important discovery in biology in
several generations.
[Footnote 25: When dealing with only a few individual cases, we do not
always find them to come out in such exact proportion; but when the
number of examples is large, the proportion is so close to these figures
that the exceptions can be entirely neglected as probably due to error
of some kind.]
There are two distinct kinds of Andalusian fowls, one pure bred black,
the other pure bred white with slight dashes of black here and there.
When these are mated, no matter which color is the father or the mother,
the next or hybrid generation are always a queer mixture of black and
white called by fanciers blue. When these blues are interbred,
one-quarter of their offspring will be white, which will prove to breed
true ever afterwards, one-quarter will be black that will breed true,
and fifty per cent. will be blue which will break up in the next
generation in the very same way as before. In this case neither white
nor black character is dominant, and accordingly we have a blending of
both in the first hybrid generation.
In guinea pigs, black color has been found to be dominant over white,
rough coat over smooth coat, and short hair over long hair. These
remarkable results following from an experimental trial of Mendelism
have stimulated hosts of investigators in all parts of the world, until
now many varieties of plants and animals have been studied for many
successive generations, already, building up a considerable literature
dealing with the subject.
Perhaps the most extensive and exact series of experiments along this
line have been carried on by Thomas Hunt Morgan and his assistants, of
Columbia University. For over five years they have been breeding the
wild fruit fly (_Drosophila ampelophila_), during which time they have
originated and observed over a hundred and twenty-five new types that
breed true according to Mendel's laws. Every part of the body has been
affected by one or another of these mutations. The wings have been
shortened, or changed in shape, or made to disappear entirely. The eyes
have been changed in color or entirely eliminated. And each of these
wonderful variations was brought about not gradually, but at _a single
step_.
Professor Morgan grows justifiably sarcastic in contrasting these
demonstrated laboratory facts with the armchair theories that have so
long and so harmfully dominated biological studies. A quotation from him
will not be out of place at this point.
"I may recall in this connection that wingless flies also arose in our
cultures by a single mutation. We used to be told that wingless insects
occurred on desert islands because those insects that had the best
developed wings had been blown out to sea. Whether this is true or not,
I will not pretend to say; but at any rate wingless insects may also
arise, not through a slow process of elimination, but at a single
step.... Formerly we were taught that eyeless animals arose in caves.
This case shows that they may also arise suddenly in glass milk bottles,
by a change in a single factor."[26]
[Footnote 26: "A Critique of the Theory of Evolution," p. 67.]
We need not be particularly concerned here with the theoretical
explanations of these facts offered in terms of the microscopic or even
the infra-microscopic components of the germ cells. Morgan seems to
make out a strong case for the theory that the chromosomes found in the
nucleus are the real ultimate units that carry the hereditary factors.
But he is quite decided in the opinion that these hereditary factors are
fixed, and are not changed from generation to generation either by
environment or by selection.[27] The important thing for us in this
connection is to get a clear idea of the results following from an
application of Mendel's laws to the old, old problem of the origin of
species, incidentally noticing how the theory associated with Darwin's
name now looks in the light of these new facts.
[Footnote 27: In human beings it has been found that the effects of
alcoholism and of syphilis are indeed transmitted according to Mendelian
law, being the two solitary examples of diseased conditions that are
thus transmitted. But they are so plainly pathologic phenomena that
there is little temptation for the advocates of Lamarckianism to use
them as proofs of their theory.]
We have hitherto been considering the results worked out by Mendel with
but one pair of contrasted characters or factors. But Mendel studied the
relation of other characters of the pea, and found among other results
that smooth seeds are dominant to wrinkled seeds, colored seeds dominant
to white, yellow color dominant to green, etc. But when a combination of
_two_ factors in each parent are put into contrast by cross breeding,
two wholly original forms (as they seemed) were sometimes produced, and
it looked as if these new kinds were really analogous to new species.
For example, he crossed tall yellow peas with dwarf green peas, with
the result that the first hybrid generation turned out to be all tall
yellows. However, in the second hybrid generation they split up
according to the law as already stated, modified by the additional
complication brought into the problem by the additional pair of factors.
For out of every sixteen plants there were nine tall yellows, three
_dwarf yellows_, three _tall greens_, and one dwarf green. It is evident
that these tall greens and dwarf yellows are really new forms; and
further experiments proved that they can be separated out or segregated
and grown as pure forms which thereafter breed true. Thus we have a very
important result for the breeder, for it enables him to work to a
definite aim and combine certain desirable characters into a single
form.
The term _mutation_, as already intimated, has been given to this
process of producing new varieties in this way. The kinds so produced
are termed _mutants_, and at first they were hailed by enthusiastic
scientists as "elementary species." De Vries in particular gave much
publicity to this idea; for he thought he had really produced a new kind
comparable in every respect to a true species as produced by nature
among wild plants. But the enthusiasm with which this applied result of
Mendel's Law was at first hailed by biologists has gradually subsided;
for it has been found that though these new forms will breed true under
certain conditions, they are nevertheless _cross-fertile with the
original forms_, and thus the circle can be _completed back again_ by a
return to the parent form, from which the new "species" can again be
produced at will with the same mathematical exactness as before.