HISTORICAL
To Gregor Mendel, monk and abbot, belongs the credit of founding the modern
science of heredity. Through him there was brought into these problems an
entirely new idea, an entirely fresh conception of the nature of living
things. Born in 1822 of Austro-Silesian parentage, he early entered the
monastery of Bruenn, and there in the seclusion of the cloister garden he
carried out with the common pea the series of experiments which has since
become so famous. In 1865 after eight years' work he published the results
of his experiments in the _Proceedings of the Natural History Society of
Bruenn_, in a brief paper of some forty pages. But brief as it is the
importance of the results and the lucidity of the exposition will always
give it high rank among the classics of biological literature. For
thirty-five years Mendel's paper remained unknown, and it was not until
1900 that it was simultaneously discovered by several distinguished
botanists. The causes of this curious neglect are not altogether without
interest. Hybridisation experiments before Mendel there had been in plenty.
The classificatory work of {9} Linnaeus in the latter half of the
eighteenth century had given a definite significance to the word species,
and scientific men began to turn their attention to attempting to discover
how species were related to one another. And one obvious way of attacking
the problem was to cross different species together and see what happened.
This was largely done during the earlier half of the nineteenth century,
though such work was almost entirely confined to the botanists. Apart from
the fact that plants lend themselves to hybridisation work more readily
than animals, there was probably another reason why zoologists neglected
this form of investigation. The field of zoology is a wider one than that
of botany, presenting a far greater variety of type and structure. Partly
owing to their importance in the study of medicine, and partly owing to
their smaller numbers, the anatomy of the vegetable was far better known
than that of the animal kingdom. It is, therefore, not surprising that the
earlier part of the nineteenth century found the zoologists, under the
influence of Cuvier and his pupils, devoting their entire energies to
describing the anatomy of the new forms of animal life which careful search
at home and fresh voyages of discovery abroad were continually bringing to
light. During this period the zoologist had little inclination or
inducement to carry on those investigations in hybridisation which were
occupying the attention of some botanists. Nor did the efforts of the
botanists afford much {10} encouragement to such work, for in spite of the
labour devoted to these experiments, the results offered but a confused
tangle of facts, contributing in no apparent way to the solution of the
problem for which they had been undertaken. After half a century of
experimental hybridisation the determination of the relation of species and
varieties to one another seemed as remote as ever. Then in 1859 came the
_Origin of Species_, in which Darwin presented to the world a consistent
theory to account for the manner in which one species might have arisen
from another by a process of gradual evolution. Briefly put, that theory
was as follows: In any species of plant or animal the reproductive capacity
tends to outrun the available food supply, and the resulting competition
leads to an inevitable struggle for existence. Of all the individuals born,
only a portion, and that often a very small one, can survive to produce
offspring. According to Darwin's theory, the nature of the surviving
portion is not determined by chance alone. No two individuals of a species
are precisely alike, and among the variations that occur some enable their
possessors to cope more successfully with the competitive conditions under
which they exist. In comparison with their less favoured brethren they have
a better chance of surviving in the struggle for existence and consequently
of leaving offspring. The argument is completed by the further assumption
of a principle of heredity, in virtue of which offspring tend to {11}
resemble their parents more than other members of the species. Parents
possessing a favourable variation tend to transmit that variation to their
offspring, to some in greater, to others in less degree. Those possessing
it in greater degree will again have a better chance of survival, and will
transmit the favourable variation in even greater degree to some of their
offspring. A competitive struggle for existence working in combination with
certain principles of variation and heredity results in a slow and
continuous transformation of species through the operation of a process
which Darwin termed natural selection.
The coherence and simplicity of the theory, supported as it was by the
great array of facts which Darwin had patiently marshalled together,
rapidly gained the enthusiastic support of the great majority of
biologists. The problem of the relation of species at last appeared to be
solved, and for the next forty years zoologists and botanists were busily
engaged in classifying by the light of Darwin's theory the great masses of
anatomical facts which had already accumulated and in adding and
classifying fresh ones. The study of comparative anatomy and embryology
received a new stimulus, for with the acceptance of the theory of descent
with modification it became incumbent upon the biologist to demonstrate the
manner in which animals and plants differing widely in structure and
appearance could be conceivably related to one another. Thenceforward the
energies of both {12} botanists and zoologists have been devoted to the
construction of hypothetical pedigrees suggesting the various tracks of
evolution by which one group of animals or plants may have arisen from
another through a long continued process of natural selection. The result
of such work on the whole may be said to have shown that the diverse forms
under which living things exist to-day, and have existed in the past so far
as palaeontology can tell us, are consistent with the view that they are
all related by the community of descent which the accepted theory of
evolution demands, though as to the exact course of descent for any
particular group of animals there is often considerable diversity of
opinion. It is obvious that all this work has little or nothing to do with
the manner in which species are formed. Indeed, the effect of Darwin's
_Origin of Species_ was to divert attention from the way in which species
originate. At the time that it was put forward his explanation appeared so
satisfying that biologists accepted the notions of variation and heredity
there set forth and ceased to take any further interest in the work of the
hybridisers. Had Mendel's paper appeared a dozen years earlier it is
difficult to believe that it could have failed to attract the attention it
deserved. Coming as it did a few years after the publication of Darwin's
great work, it found men's minds set at rest on the problems that he raised
and their thoughts and energies directed to other matters. {13}
Nevertheless one interesting and noteworthy attempt to give greater
precision to the term heredity was made about this time. Francis Galton, a
cousin of Darwin, working upon data relating to the breeding of Basset
hounds, found that he could express on a definite statistical scheme the
proportion in which the different colours appeared in successive
generations. Every individual was conceived of as possessing a definite
heritage which might be expressed as unity. Of this, 1/2 was on the average
derived from the two parents (_i.e._ 1/4 from each parent), 1/4 from the
four grandparents, 1/8 from the eight great-grandparents, and so on. _The
Law of Ancestral Heredity_, as it was termed, expresses with fair accuracy
some of the statistical phenomena relating to the transmission of
characters in a mixed population. But the problem of the way in which
characters are distributed from gamete to zygote and from zygote to gamete
remained as before. Heredity is essentially a physiological problem, and
though statistics may be suggestive in the initiation of experiment, it is
upon the basis of experimental fact that progress must ultimately rest. For
this reason, in spite of its ingenuity and originality, Galton's theory and
the subsequent statistical work that has been founded upon it failed to
give us any deeper insight into the nature of the hereditary process.
While Galton was working in England the German zoologist August Weismann
was elaborating the complicated {14} theory of heredity which eventually
appeared in his work on _The Germplasm_ (1885), a book which will be
remembered for one notable contribution to the subject. Until the
publication of Weismann's work it had been generally accepted that the
modifications brought about in the individual during its lifetime, through
the varying conditions of nutrition and environment, could be transmitted
to the offspring. In this biologists were but following Darwin, who held
that the changes in the parent resulting from increased use or disuse of
any part or organ were passed on to the children. Weismann's theory
involved the conception of a sharp cleavage between the general body
tissues or somatoplasm and the reproductive glands or germplasm. The
individual was merely a carrier for the essential germplasm whose
properties had been determined long before he was capable of leading a
separate existence. As this conception ran counter to the possibility of
the inheritance of "acquired characters," Weismann challenged the evidence
upon which it rested and showed that it broke down wherever it was
critically examined. By thus compelling biologists to revise their ideas as
to the inherited effects of use and disuse, Weismann rendered a valuable
service to the study of genetics and did much to clear the way for
subsequent research.
A further important step was taken in 1895, when Bateson once more drew
attention to the problem of the origin {15} of species, and questioned
whether the accepted ideas of variation and heredity were after all in
consonance with the facts. Speaking generally, species do not grade
gradually from one to the other, but the differences between them are sharp
and specific. Whence comes this prevalence of discontinuity if the process
by which they have arisen is one of accumulation of minute and almost
imperceptible differences? Why are not intermediates of all sorts more
abundantly produced in nature than is actually known to be the case?
Bateson saw that if we are ever to answer this question we must have more
definite knowledge of the nature of variation and of the nature of the
hereditary process by which these variations are transmitted. And the best
way to obtain that knowledge was to let the dead alone and to return to the
study of the living. It was true that the past record of experimental
breeding had been mainly one of disappointment. It was true also that there
was no tangible clue by which experiments might be directed in the present.
Nevertheless in this kind of work alone there seemed any promise of
ultimate success.
A few years later appeared the first volume of de Vries' remarkable book on
_The Mutation Theory_. From a prolonged study of the evening primrose
(_Oenothera_) de Vries concluded that new varieties suddenly arose from
older ones by sudden sharp steps or mutations, and not by any process
involving the gradual accumulation of minute {16} differences. The number
of striking cases from among widely different plants which he was able to
bring forward went far to convincing biologists that discontinuity in
variation was a more widespread phenomenon than had hitherto been
suspected, and not a few began to question whether the account of the mode
of evolution so generally accepted for forty years was after all the true
account. Such in brief was the outlook in the central problem of biology at
the time of the rediscovery of Mendel's work.
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