The attribution attempted here of the essential difference in the development of hybrids to _a permanent or temporary union_ of the differing cell elements can, of course, only claim the value of an hypothesis for which the lack of definite data offers a wide field. Some justification of the opinion expressed lies in the evidence afforded by _Pisum_ that the behaviour of each pair of differentiating characters in hybrid union is independent of the other differences between the two original plants, and, further, that the hybrid produces just so many kinds of egg and pollen cells as there are possible constant combination forms. The differentiating characters of two plants can finally, however, only depend upon differences in the composition and grouping of the elements which exist in the foundation-cells [fertilised ova] of the same in vital interaction[47]. [47] “_Welche in den Grundzellen derselben in lebendiger Wechselwirkung stehen._” Even the validity of the law formulated for _Pisum_ requires still to be confirmed, and a repetition of the more important experiments is consequently much to be desired, that, for instance, relating to the composition of the hybrid fertilising cells. A differential [element] may easily escape the single observer[48], which although at the outset may appear to be unimportant, may yet accumulate to such an extent that it must not be ignored in the total result. Whether the variable hybrids of other plant species observe an entire agreement must also be first decided experimentally. In the meantime we may assume that in material points a difference in principle can scarcely occur, since the unity in the developmental plan of organic life is beyond question. [48] “_Dem einzelnen Beobachter kann leicht ein Differenziale entgehen._” In conclusion, the experiments carried out by Kölreuter, Gärtner, and others with respect to _the transformation of one species into another by artificial fertilisation_ merit special mention. A special importance has been attached to these experiments, and Gärtner reckons them among “the most difficult of all in hybridisation.”