6. The existence of constitutional units seems otherwise necessarily implied. I refer to the fact that no organism is a homogeneous mean between its parents but consists of a mixture of parts, some following one parent and some the other. Among illustrations of this the most conspicuous are those yielded by the variously-mixed colours of hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display these mixtures: colours in one place like the mother and in another place like the father. As the internal organs are invisible, and as visible organs have indefinite shapes and graduate indefinitely into adjacent ones, the mixture of traits is elsewhere less conspicuous; but occasional marked cases (especially in malformations) leave no doubt that it pervades the entire organism.
This peculiarity of transmission seems necessarily to imply that there are distinct units derived from the two parents, and that in the course of development there is more or less segregation of them—those of the one origin predominating so far in some places as to give special likeness to one parent, and those derived from the other doing the like in other places. All which interpretation is impossible unless the hypothesis of constitutional units be admitted.
7. I come at length to the special evidence referred to at the outset. It is evidence of the same nature as that just assigned, but carried to a higher stage. It is furnished not by the segregation of traits derived from two parents of the same variety, but is furnished by the segregation of traits derived from parents of different varieties. In articles on “Bud Variations or Sports” (Gardener’s Chronicle, 1891) Dr. Masters gives various examples of the separation or unmixing of ancestral constitutions. Mr. Noble formed a hybrid between Clematis Jackmani and C. patens. One of these varieties flowers in the autumn on new wood, while the other flowers in the spring on old wood; and the result is that flowers of two kinds, quite unlike, are produced at different parts of the year, and that by pruning so as to cut away one or other set of shoots, the plant may be made to produce exclusively for the time being one or other sort of flower.
“Another very interesting case of unmixing, or, if it be preferred, of partial mixture, is afforded by Neubert’s Berberis. This is a hybrid between the evergreen pinnate-leaved Mahonia and the deciduous simple-leaved Berberis vulgaris, and it bears leaves some of which are intermediate in appearance, while others are much like those of one or other of its parents.
“A not uncommon illustration of a similar kind, is the production of a Peach and a Nectarine on the same branch, and we have just learnt from Canon Ellacombe that some of the Berlin Hellebores show evidence of their hybrid nature by occasionally producing foliage [and flowers?] of the two parents separately from the same root-stock.
“In addition to the cases given above, we may here cite a few more which have come under our notice, such as a Chrysanthemum, half the florets of which are of one colour, half of another. A hybrid Calanthe, showing a similar piebald variation, is shown in Fig. 14. A very curious case was that of the Narcissus received from Mr. Walker, and in which flowers of two distinct varieties sprang from the same bulb. Grapes not uncommonly show their crossed origin by presenting a striped appearance, one stripe being of one colour, one of another, as may also be seen in the Orange, Apple, Lemon, and Currant.”
Thus, however the germ-plasm is constituted its essential components cannot be all alike. Before there can be this dissociation of ancestral characters, there must be in the germ-plasm different elements capable of being dissociated. This single fact seems to compel us to assume constitutional units.