16 THE QUANTITATIVE METHOD IN BIOLOGY 



have been propagated by means of cuttings, layers, tubers and 

 other methods of vegetative propagation. It is possible to 

 obtain in this way an unlimited number of specimens which are 

 all characterized by the new properties and seem to belong to 

 a peculiar species or subspecies. Many so-called horticultural 

 species have their origin in a bud- variation. 



EXAMPLES: Mr KNIGHT states that a tree of the yellow magnum 

 bonum plum, forty years old, which had always borne ordinary fruit, produced 

 a branch which yielded red magnum bonums (Darwin, loc. cit., p. 399). The 

 black or purple Frontignan (grape) in one case produced during two successive 

 years (and no doubt permanently) spurs which bore white Frontignan grapes 

 (DARWIN, loc. cit., p. 399). Kemp's potato is properly white, but a plant 

 in Lancashire produced two tubers which were red and two which were white. 

 The red kind was propagated in the usual manner by eyes, and kept true to its 

 new colour, and, being found a more productive variety, soon became widely 

 known under the name of Taylor's fortyfold (DARWIN, loc. cit., p. 410). 



The pseudo-species produced by bud-variation (we may call 

 them hud-species or bud-subspecies) differ from the true species 

 (seed-species) because their characteristic properties are not 

 true to seed, which means that they are not transmissible by 

 sexual reproduction : as a rule, the seeds of the bud-species 

 produce specimens in which the new properties have dis- 

 appeared. The bud-species are plastic just as the seed-species. 



It may be suggested that a bud-species is produced by a 

 qualitative change of the living mixture (§ 8a) localized in 

 a bud and transmissible only by vegetative multiplication. In 

 other words, a bud- variation might be looked upon as being a 

 non seed-fixed ^ mutation. 



In a hybrid specimen we observe, in general, a combination of a number of 

 properties (characters) borrowed from the parents. This combination is 

 ordinarily transmitted only to a part of the children (seedlings) : the seminal 

 offspring of a given hybrid is very variable, the properties under consideration 

 being combined in various ways among the children (as a consequence of the 

 principle of segregation). The hereditary transmission of the characteristic 

 combination of a given hybrid by sexual reproduction is therefore impossible 

 in horticultural practice, especially in those cases in which the combined 

 properties are numerous — for instance, when the hybrid is a complex one, 

 produced by successive crossings between several species. ^ 



It is, however, possible to obtain from a hybrid an unlimited number of 

 specimens similar to the parent by means of vegetative multiplication, the 

 characteristic combination being transmitted in this way. Many so-called 

 horticultural species consist in reality of the vegetative offspring of one hybrid 

 specimen . 



Between bud-species and hybrids a certain analogy exists in respect to 

 heredity : in both cases the characters vanish by seminal reproduction and 

 are transmitted by vegetative multiplication. 



Concerning constant hybrids, see § 105. 



REMARK : It is possible that bud-species (or subspecies) 

 exist in the state of nature. A bud-variation might easily be 



^ This is the translation of the Dutch word zaadvast {zaad = seed and vast = 

 fixed). 



2 See, for instance, the complex hybrids in the genus Gladiolus mentioned by 

 DE VRIES (Mutationstheorie). 



