Species and Varieties, Their Origin by Mutation | Page 6

to indicate the possible and most probable
ways, but no more. On the other hand the accumulation of fluctuations
does not transgress relatively narrow [9] limits as far as the present
methods of selection go. But the question remains to be solved,
whether our methods are truly the right ones, and whether by the use of
new principles, new results might not cause the balance of opinion to
favor the opposite side.
Of late, a thorough and detailed discussion of the opposing views has
been given by Morgan in his valuable book on evolution and adaptation.
He has subjected all the proposed theories to a severe criticism both on
the ground of facts and on that of their innate possibility and logical
value. He decides in favor of the mutation theory. His arguments are
incisive and complete and wholly adapted to the comprehension of all
intelligent readers, so that his book relieves me entirely of the necessity
of discussing these general questions, as it could not be done in a better
or in a clearer way.
I intend to give a review of the facts obtained from plants which go to
prove the assertion, that species and varieties have originated by
mutation, and are, at present, not known to originate in any other way.
This review consists of two parts. One is a critical survey of the facts of
agricultural and horticultural breeding, as they have accumulated since
the time of Darwin. This body of evidence is to be combined with some
corresponding experiments [10] concerning the real nature of species in
the wild state. The other part rests on my own observations and
experiments, made in the botanical garden of the University of
Amsterdam.
For many years past I have tried to elucidate the hereditary conditions
of species and varieties, and the occasional occurrence of mutations,
that suddenly produce new forms.
The present discussion has a double purpose. On one side it will give
the justification of the theory of mutations, as derived from the facts
now at hand. On the other hand it will point out the deficiencies of
available evidence, and indicate the ways by which the lacunae may
gradually be filled. Experimental work on heredity does not require
vast installments or costly laboratory equipment. It demands chiefly

assiduity and exactitude. Any one who has these two qualities, and who
has a small garden at his disposal is requested to take part in this line of
investigation.
In order to observe directly the birth of new forms it is necessary, in the
first place, to be fully clear concerning the question as to what forms
are to be expected to arise from others, and before proceeding to a
demonstration of the origin of species, it is pertinent to raise the
question as to what constitutes a species.
Species is a word, which always has had a [11] double meaning. One is
the systematic species, which is the unit of our system. But these units
are by no means indivisible. Long ago Linnaeus knew them to be
compound in a great number of instances, and increasing knowledge
has shown that the same rule prevails in other instances. Today the vast
majority of the old systematic species are known to consist of minor
units. These minor entities are called varieties in systematic works.
However, there are many objections to this usage. First, the term
variety is applied in horticulture and agriculture to things so widely
divergent as to convey no clear idea at all. Secondly, the subdivisions
of species are by no means all of the same nature, and the systematic
varieties include units the real value of which is widely different in
different cases. Some of these varieties are in reality as good as species,
and have been "elevated," as it is called by some writers, to this rank.
This conception of the elementary species would be quite justifiable,
and would at once get rid of all difficulties, were it not for one practical
obstacle. The number of the species in all genera would be doubled and
tripled, and as these numbers are already cumbersome in many cases,
the distinction of the native species of any given country would lose
most of its charm and interest.
[12] In order to meet this difficulty we must recognize two sorts of
species. The systematic species are the practical units of the
systematists and florists, and all friends of wild nature should do their
utmost to preserve them as Linnaeus has proposed them. These units
however, are not really existing entities; they have as little claim to be
regarded as such as genera and families. The real units are the
elementary species; their limits often apparently overlap and can only
in rare cases be determined on the sole ground of field observations.