616 APPENDIX F. 



cal change, is great where the unlikeness is great and diminishes 

 with the approach towards likeness. 



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 inter- 

 pretation 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 segre- 

 gation 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 par- 

 tial 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 



