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 numbers. It is obvious that, properly speaking, the whole family of a mutated individual, including all its nearer and more remote relatives, should be known and recorded. Hence pedigree-book-keeping must become the general rule. Subordinate to this are two further points, which should likewise be stated here. One pertains to the pure or hybrid nature of the original strain, and the other to the life-conditions and all other external influences. It is manifest that a complete understanding of a mutation depends upon full information upon these points. All experiments must have a beginning. The starting-point may be a single individual, or a small group of plants, or a lot of seeds. In many cases the whole previous history is obscure, but sometimes a little historical evidence is at hand. Often it is evident that the initial material belongs to a pure species, but with respect to the question of elementary species it is [25] not rarely open to doubt. Large numbers of hybrid plants and hybrid races are in existence, concerning the origin of which it is impossible to decide. It is impossible in many instances to ascertain whether they are of hybrid or of pure origin. Often there is only one way of determining the matter; it is to guess at the probable parents in case of a cross and to repeat the cross. This is a point which always requires great care in the interpretation of unusual facts.