Page:Mendel's principles of heredity; a defence.pdf/120

 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 &#9792; × H. aurantiacum &#9794; 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 &#9792; × H. aurantiacum &#9794; constitute approximately intermediate forms which do not agree in certain characters. On the contrary in H. Auricula &#9792; × H. aurantiacum &#9794; and in H. Auricula &#9792; × H. pratense &#9794; 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 &#9792; × H. aurantiacum &#9794; may be described as fully fertile; H. præaltum &#9792; × H. flagellare &#9794; as fertile; H. præultum &#9792; × H. aurantiacum &#9794; and H. Auricula &#9792; × H. pratense &#9794; as