Species and Varieties, Their Origin by Mutation | Page 9

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 is as simple in theory, as it is difficult to carry out in practice. First of all comes the book-keeping. The parents, grandparents and previous ancestors must be known individually. Accounts of them must be kept under two headings. A full description of their individual character and peculiarities must always be available on the one hand, and on the other, all facts concerning their hereditary [24] qualities. These are to be deduced from the composition of the progeny, and in order to obtain complete evidence on this point, two successive generations are often required. The investigation must ascertain the average condition of this offspring and the occurrence of any deviating specimens, and for both purposes it is necessary to cultivate them in relatively large