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.
If we compare the individual characters of the hybrids with the corresponding characters of the two parent types, we find that they sometimes present intermediate structures, but are sometimes so near to one of the parent characters that the [corresponding] character of the other has receded considerably or almost evades observation. So, for instance, we see in one of the two forms of H. Auricula ♀ x H. aurantiacum ♂ pure yellow disc-florets; only the petals of the marginal florets are on the outside tinged with red to a scarcely noticeable degree: in the other on the contrary the colour of these florets comes very near to H. aurantiacum, only in the centre of the disc the orange red passes into a deep golden-yellow. This difference is noteworthy, for the flower-colour in Hieracium has the value of a constant character. Other similar cases are to be found in the leaves, the peduncles, &c.
If the hybrids are compared with the parent types as regards the sum total of their characters, then the two forms of H. præaltum ♀ x H. aurantiacum ♂ constitute approximately intermediate forms which do not agree in certain characters. On the contrary in H. Auricula ♀ x H. aurantiacum ♂ and in H. Auricula ♀ x H. pratense ♂ we see the forms widely divergent, so that one of them is nearer to the one and the other to the other parental type, while in the case of the last-named hybrid there is still a third which is almost precisely intermediate between them.
The conviction is then forced on us that we have here only single terms in an unknown series which may be formed by the direct action of the pollen of one species on the egg-cells of another.
2. With a single exception the hybrids in question form seeds capable of germination. H. echioides ♀ x H. aurantiacum ♂ may be described as fully fertile; H. præaltum ♀ x H. flagellare ♂ as fertile; H. præaltum ♀ x H. aurantiacum ♂ and H. Auricula ♀ x H. pratense ♂ as partially fertile; H. Auricula ♀ x H. Pilosella ♂ as slightly fertile, and H. Auricula ♀ x H. aurantiacum ♂ as unfertile. Of the two forms of the last named hybrid, the red-flowered one was completely sterile, but from the yellow-flowered one a single well-formed seed was obtained. Moreover it must not pass unmentioned that among the seedlings of the partially fertile hybrid H. præaltum ♀ x H. aurantiacum ♂ there was one plant which possessed full fertility.
[3.] As yet the offspring produced by self-fertilisation of the hybrids have not varied, but agree in their characters both with each other and with the hybrid plant from which they were derived.
From H. præaltum ♀ x H. flagellare ♂ two generations have flowered; from H. echioides ♀ x H. aurantiacum ♂, H. præaltum ♀ x H. aurantiacum ♂, H. Auricula ♀ x H. Pilosella ♂ one generation in each case has flowered.