OBSERVABLE PROPERTIES OF EACH SPECIES 17 



multiplied in the very numerous species {Ficaria ranunculoides, 

 Ajiiga reptans, Potentilla anserina, etc., etc.) which ordinarily 

 multiply by vegetative means. ^ If natural bud-species really 

 occur, it would be impossible to distinguish them from seed- 

 species without experiment. One would expect to find eventu- 

 ally numerous specimens of them in narrowly limited localities. 



§ 21.— CONTINUOUS AND DISCONTINUOUS VARIA- 

 TION. — Confusion exists in the use of the terms continuous 

 and discontinuous variation. 



The variation of a property is called continuous when all 

 possible transitions exist between the extremes. When we 

 measure, for instance, the length of the spike of a number of 

 specimens of one subspecies (so-called race) of the rye, and set 

 them in order into a series according to their length, we see 

 that the measured property increases gradually from the 

 shortest to the longest one : there are neither gaps nor jumps 

 in the series (Dr C. DE BRUYKER). 



If, on the contrary, a gap is observed in the series, by which 

 the measured specimens can be divided into two groups, the 

 variation is called discontinuous. 



In a similar way we say that discontinuity exists when two 

 groups of specimens differ from one another in a pair of 

 properties which are distinctly different. EXAMPLE : In 

 Lychnis diurna {Melandryum roseum) the petals are pink. In 

 Lychnis vespertina (Melandryum album) the petals are white. 



Those two kinds of discontinuity must be distinguished from 

 one another. This subject is expounded more completely in 

 § 130. (See also § 112.) In the present paragraph I content 

 myself with calling attention to the very important fact that 

 discontinuity (in the values of one property) sometimes occurs 

 in a group of specimens between which no specific difference 

 whatever exists, and that, on the other hand, continuity is 

 often observed although the specimens under consideration 

 belong to two (or even more) distinctly different specific forms. 



§ 22.— ADAPTATION. ACCOMMODATION.— There is a 

 relation between variation and so-called adaptation. A strict 

 distinction ought to be made between individual adaptation 

 (accommodation), which is the consequence of plasticity, and 

 the HYPOTHETICAL adaptation of the species, which is 

 supposed to be produced by natural selection (Darwinism) 

 or by the hereditary transmission and fixation of individual 

 adaptation (Neo-Lamarckism). (See on this subject §§ 44-45.) 



§ 23.— VARIATION UNDER CULTIVATION OR DO- 

 MESTICATION AND IN THE STATE OF NATURE.— The 



^ A rather large number of indigenous species rarely bear seed. 

 B 



