92^ ORIGIN OF SPECIES. 



six to twenty in number ; they may be white, yellow, red, orange, violet, streaked 

 with black, blue, or copper- red; their weight varies sevenfold; their form also varies 

 extremely; there are varieties with three kinds of fruit of different form and colour 

 on one rachis ; and a great number of other differences also occur \ These 

 instances may suffice to show to what an extent the amount of deviation of the 

 varieties of a primitive form may increase under cultivation ^. 



It is much more difficult, and to a great extent impossible, to prove directly to 

 what extent the variation of wild forms can increase without cultivation, because 

 historical evidence is in this case generally impossible, or can only be obtained indi- 

 rectly or conjecturally. But since the laws of variation are unquestionably the same 

 in the case of wild as of cultivated plants-^al though they operate in the two cases 

 under different conditions — we may for the present at least assume as probable that 

 plants vary as greatly in the wild as in the cultivated state. We shall however in the 

 sequel have to examine a number of weighty considerations which lead to the con- 

 clusion that variation has produced infinitely greater effects in originating the various 

 wild forms of plants than those which we perceive in cultivated varieties ^. 



The variation of cultivated plants shows that there is only one cause for the 

 internal and for the external hereditary resemblance between different plants, and 

 that this cause is the common origin of similar forms from one and the same 

 ancestral form. When we meet with corresponding phenomena in wild forms, and 

 when we find that with them dissimilar forms are connected by a series of inter- 

 mediate forms, just as we find to be the case between the primitive forms of culti- 

 vated plants and their most abnormal varieties, we are forced to the conclusion that 

 in wild plants also a similar affinity is the only cause of resemblance. The extra- 

 ordinarily numerous forms, for example, of the widely distributed genus Hieraciuvt 

 present phenomena similar in many respects to those of cultivated Gourds, Cab- 

 bages, &c. In addition to a number of forms which are considered to be species, 

 there are a still greater number of intermediate forms, some of which only are 

 hybrids, the greater part perfectly fertile varieties. Nageli ^, who has made this genus 

 the subject of close study, says : — * If an attempt is made to unite into a single 

 species all the types which are connected by perfectly fertile transitional forms, we 

 should find only three species of native Hieracia, which have been erected by some 

 authors into distinct genera : — Pilosella (Piloselloidese), Hieracium {Archieracium), 

 and Chlorocrepis [H. staiicifolium). Between these three groups we have, at least 

 in Europe, no transitional forms ; hybrids between Piloselloideae and Archieraciujn 



^ See Darwin, /. c. vol. I. p. 365, and Metzger, /. c. p. 207. No great value with reference to 

 variation and the constancy of varieties must be set on the result of experiments on cultivated plants, 

 since the possibility of hybridisation was not excluded. Some varieties of Maize appear to hybridise 

 with difficulty. 



^ Further material will be found collected in Darwin's and Metzger's works already quoted, and 

 in De Candolle, Geographic botanique, Paris 1855. 



^ [H. Hoffmann gives in the Bot. Zeit. for April 27 and May i, 1874, an account of an inter- 

 esting series of experiments on the extent^ to which the characters which distinguish the allied 

 species Papaver Rhceas and dubium and Phafeoh/s vulgaris and multijlorus can be made to vary by 

 cultivation, and on the tendency of the cultivated varieties to revert to the parent-form. See also 

 his 'Riickblick auf meine Variations- Versuche von 1855-1880,' Bot. Zeitg. 1881.] 



* Sitzungsberichte der kon, bayer, Akad. der Wiss. March 10, 1866. 



