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.