n8 PLANT PHYSIOLOGY 



having bright pink flowers with red streaks. In the second generation we 

 find, on the whole, eleven different kinds of flowers which show transitions 

 from white to red or white to yellow. The pigment appears in solitary streaks 

 on a white ground, as a ' pure ' self colour of brighter or darker shade, or 

 finally as dark streaks on a brighter ground. Since we know that in races 

 which follow the law of prevalency a character may appear, externally, to be 

 lost in the first generation, but makes its appearance again in the next, we 

 must assume that in the case of Mirabilis also the red initials lie latent both 

 in the white and yellow forms, and we are strengthened in this belief when 

 we learn that both forms arise from a red type. Similarly, the above-men- 

 tioned species of Datura are doubtless descendants of pigmented forms. In 

 a word, we have here to deal with atavistic characters or reversions (comp. 

 CORRENS, 1905 a). It is also probable that characters which make their appear- 

 ance for the first time arise as initials, and that these may most easily show 

 themselves in visible forms by appropriate crossing. 



The large number of so-called new characters which have been observed 

 in hybrids spring, no doubt, from combinations of old characters and from 

 atavistic initials which become active. Other differences between hybrids 

 and their parents can scarcely be regarded as distinct 'characters'. Thus 

 hybrids are often distinguished from the pure races by their power of ' vege- 

 tative growth '. This energy may be feebler than that of the parent plants 

 when the parents are not closely related forms. In this case the seeds germinate 

 badly, and the seedlings are difficult to rear. Or and this is particularly 

 applicable to hybrids between nearly-related races they are remarkable for 

 their size, rapid growth, early blooming, free flowering, longer period of life, 

 great capacity for multiplication, abnormal size of individual organs, and 

 similar characters (FocKE, 1881, p. 475). If, for instance, the hybrid Datura 

 Tatula x D. Stramonium attains a height of two metres, while the parents only 

 attain a height of about one metre, we may say that the hybrid has acquired 

 a new character, nevertheless it is only a quantitative and not a qualitative 

 variation; such as we might obtain otherwise, e.g. by over-nutrition in seed- 

 formation or good manuring in germination. One may look just as little upon 

 the increased growth-energy of the hybrid as on the other, at all events, frequent 

 characteristic, its diminished fertility, as a serious objection to the view that 

 hybrids show no new characters. This diminished fertility is usually mani- 

 fested in partially or completely unfertile pollen, more rarely in immature 

 ovules. For this reason it is often possible to rear fruits and seeds only by 

 pollinating from the parental line, although there is a class of hybrids whose 

 ovules are quite fertile with their own pollen (Salix, Hieracium). At the extreme 

 limit of sterility are many species of Rhododendron, Epilobium, &c., hybrids 

 of which, in general, do not even form flowers. 



Before proceeding to study hybrids from an entirely different aspect, we 

 will inquire what criticisms as to the phenomena of fertilization are suggested 

 by the results hitherto arrived at from experimental hybridizing. 



376, 1. ii, for II, 180]. read II, Chapter xix]. 



1. 20, after polymorphic, read Possibly under certain circumstances both 

 may take place. Since further segregation may, without doubt, occur not 

 only among hybrids but also in fertilization of similar races, it is possible 

 that a fusion which has occurred may break down again in its units in the 

 offspring. 



Heredity, the laws of which we have become acquainted with in hybrids, 

 is a peculiarity of organisms which shows itself both in the simplest form of 

 reproduction, viz. division, as in the most complicated sexual act. 



1. 45 P. 381, 1. 36, for It is impossible . . . kind of hybrid, read The pheno- 



