Regarding the question whether and to what extent hybridisation plays a part in the production of this wealth of forms, we find very various and conflicting views held by leading botanists. While some of them maintain that this phenomenon has a far-reaching influence, others, for example, Fries, will have nothing to do with hybrids in _Hieracia_. Others take up an intermediate position; and while granting that hybrids are not rarely formed between the species in a wild state, still maintain that no great importance is to be attached to the fact, on the ground that they are only of short duration. The [suggested] causes of this are partly their restricted fertility or complete sterility; partly also the knowledge, obtained by experiment, that in hybrids self-fertilisation is always prevented if pollen of one of the parent-forms reaches the stigma. On these grounds it is regarded as inconceivable that _Hieracium_ hybrids can constitute and maintain themselves as fully fertile and constant forms when growing near their progenitors. The question of the origin of the numerous and constant intermediate forms has recently acquired no small interest since a famous _Hieracium_ specialist has, in the spirit of the Darwinian teaching, defended the view that these forms are to be regarded as [arising] from the transmutation of lost or still existing species. From the nature of the subject it is clear that without an exact knowledge of the structure and fertility of the hybrids and the condition of their offspring through several generations no one can undertake to determine the possible influence exercised by hybridisation over the multiplicity of intermediate forms in _Hieracium_. The condition of the _Hieracium_ hybrids in the range we are concerned with must necessarily be determined by experiments; for we do not possess a complete theory of hybridisation, and we may be led into erroneous conclusions if we take rules deduced from observation of certain other hybrids to be Laws of hybridisation, and try to apply them to _Hieracium_ without further consideration. If by the experimental method we can obtain a sufficient insight into the phenomenon of hybridisation in _Hieracium_, then by the help of the experience which has been collected respecting the structural relations of the wild forms, a satisfactory judgment in regard to this question may become possible. Thus we may express the object which was sought after in these experiments. I venture now to relate the very slight results which I have as yet obtained with reference to this object. 1. Respecting the structure of the hybrids, we have to record the striking phenomenon that the forms hitherto obtained by similar fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three individuals, while as to the remainder only one of each has been obtained.