80 DARWINISJI CHAP. 



subsequently shows that, in Xanthoxyhim monogynum, ' it 

 often happens that on the same plant, on the same panicle, 

 Ave find flowers with one or with two ovaries ;' and that this is 

 an important character is shown by the Rutaceae (to which 

 Xanthoxylum belongs), being placed in a group of natural 

 orders characterised by having a solitary ovary." 



" De Candolle has divided the Cruciferje into five sub-orders 

 in accordance with the position of the radicle and cotyledons, 

 yet Mons. T. Gay (Ann. des Scien. Nat., ser. i. torn. vii. p. 389) 

 found in sixteen seeds of Petrocallis Pyrenaica the form of the 

 embryo so uncertain that he could not tell whether it ought 

 to be placed in the sub-orders ' Pleurorhiz6e ' or 'Notorhizee'; 

 so again (p. 400) in Cochlearia saxatilis M. Gay examined 

 twenty-nine embryos, and of these sixteen were vigorously 

 ' pleurorhiz6es,' nine had characters intermediate between 

 pleuro- and notor- hizt^es, and four Avere pure notorhizees." 



" M. Raspail asserts {Ann. des Scien. Nat, ser. i. tom. v. p. 



440) that a grass (Nostus Borbonicus) is so eminently variable 



in its floral organisation, that the varieties might serve to 



make a family Avith sufficiently numerous genera and tribes — 



a remark which shoAvs that important organs must be here 



variable." 



Species which vary little. 



The preceding statements, as to the great amount of 

 variation occurring in animals and plants, do not prove 

 that all species vary to the same extent, or even A^ary at 

 all, but, merely, that a considerable number of species in 

 every class, order, and family do so vary. It A\all have 

 been observed that the examples of great variability have 

 all been taken from common species, or species which have 

 a Avide range and are abundant in individuals. Noav Mr. 

 DarAvin concludes, from an elaborate examination of the floras 

 and faunas of several distinct regions, that common, Avide 

 ranging species, as a rule, vary most, AA'hile those that are 

 confined to special districts and are therefore comparatively 

 limited in number of indiAiduals vary least. By a similar 

 comparison it is shown that sj^ecies of large genera vary more 

 than species of small genera. These facts explain, to some 

 extent, Avhy the opinion has been so prevalent that A^ariation 

 is A^ery limited in amount and exceptional in character. For 



