Species and Varieties, Their Origin by Mutation | Page 9

nature little is known about improvement in this way, but
[19] local adaptations with slight changes of the average character in

separate localities, seem to be of quite normal occurrence.
A new method of individual selection has been used in recent years in
America, especially by W.M. Hays. It consists in judging the hereditary
worth of a plant by the average condition of its offspring, instead of by
its own visible characters. If this determination of the "centgener
power," as Hays calls it, should prove to be the true principle of
selection, then indeed the analogy between natural and artificial
selection would lose a large part of its importance. We will reserve this
question for the last lecture, as it pertains more to the future, than to our
present stock of knowledge.
Something should be said here concerning hybrids and hybridism. This
problem has of late reached such large proportions that it cannot be
dealt with adequately in a short survey of the phenomena of heredity in
general. It requires a separate treatment. For this reason I shall limit
myself to a single phase of the problem, which seems to be
indispensable for a true and at the same time easy distinction between
elementary species and retrograde varieties. According to accepted
terminology, some crosses are to be considered as unsymmetrical,
while others are symmetrical. The first are one-sided, [21] some
peculiarity being found in one of the parents and lacking in the other.
The second are balanced, as all the characters are present in both
parents, but are found in a different condition. Active in one of them,
they are concealed or inactive in the other. Hence pairs of contrasting
units result, while in unbalanced crosses no pairing of the particular
character under consideration is possible. This leads to the principal
difference between species and varieties, and to an experimental
method of deciding between them in difficult and doubtful cases.
Having thus indicated the general outlines of the subjects I shall deal
with, something now may be said as to methods of investigation.
There are two points in which scientific investigation differs from
ordinary pedigree-culture in practice. First the isolation of the
individuals and the study of individual inheritance, instead of averages.
Next comes the task of keeping records. Every individual must be
entered, its ancestry must be known as completely as possible, and all
its relations must be noted in such a form, that the most complete
reference is always possible. Mutations may come unexpectedly, and
when once arisen, their parents and grand-parents should be known.

Records must be available which will allow of a most complete
knowledge of the whole ancestral [22] line. This, and approximately
this only, is the essential difference between experimental and
accidental observation.
Mutations are occurring from time to time in the wild state as well as in
horticulture and agriculture. A selection of the most interesting
instances will be given later. But in all such cases the experimental
proof is wanting. The observations as a rule, only began when the
mutation had made its appearance. A more or less vague remembrance
about the previous state of the plants in question might be available,
though even this is generally absent. But on doubtful points, concerning
possible crosses or possible introduction of foreign strains, mere
recollection is insufficient. The fact of the mutation may be very
probable, but the full proof is, of course, wanting. Such is the case with
the mutative origin of Xanthium commune Wootoni from New Mexico
and of Oenothera biennis cruciata from Holland. The same doubt
exists as to the origin of the Capsella heegeri of Solms-Laubach, and of
the oldest recorded mutation, that of Chelidonium laciniatum in
Heidelberg about 1600.
First, we have doubts about the fact itself. These, however, gradually
lose their importance in the increasing accumulation of evidence.
Secondly, the impossibility of a closer [23] inquiry into the real nature
of the change. For experimental purposes a single mutation does not
suffice; it must be studied repeatedly, and be produced more or less
arbitrarily, according to the nature of the problems to be solved. And in
order to do this, it is evidently not enough to have in hand the mutated
individual, but it is indispensable to have also the mutable parents, or
the mutable strain from which it sprang.
All conditions previous to the mutation are to be considered as of far
higher importance than all those subsequent to it.
Now mutations come unexpectedly, and if the ancestry of an accidental
mutation is to be known, it is of course necessary to keep accounts of
all the strains cultivated. It is evident that the required knowledge
concerning the ancestry of a supposed mutation, must necessarily
nearly all be acquired from the plants in the experimental garden.
Obviously this rule