INVASION 233 



Darwin^, in speaking- of convergence, has said: "If two species, belong- 

 ing to two distinct though allied genera, had both produced a large number 

 of new and divergent forms, it is conceivable that these might approach each 

 other so closely that they would have all to be classified under the same 

 genus ; and thus the descendants of two distinct genera would converge 

 into one." The application of this statement to species would at once show 

 the possibility of polyphylesis in the latter, and a further examination of 

 tlie matter will demonstrate its probability. It is perfectly evident that a 

 species may be split into two or more forms by varying the conditions, let 

 us say of water-content, and that the descendants of these forms may again 

 be changed into the parent type by reversing the process. This has, in 

 fact, been done experimentally. Since it is admittedly impossible to draw 

 any absolute line between forms, varieties, and species, it is at once clear that 

 two distinct though related species, especially if they are plastic, may 

 be caused to converge in such a way that the variants may constitute 

 a new and homogeneous species. This may be illustrated by a concrete 

 case at present under investigation. Kuhnistera purpurea differs from 

 K. Candida in being smaller, in having fewer, smaller, and more narrow 

 leaflets, and a globoid spike of purple flowers in place of an elongated 

 one of white flowers ; in a word, it is more xerophytic. This conclusion 

 is completely corroborated by its occurrence. On dozens of slopes 

 examined, Kuhnistera purpurea has never been found mingling with K. 

 Candida on lower slopes, except where an accident of the surface has resulted 

 in a local decrease of water-content. The experiment as conducted is a 

 simple one, consisting merely in sowing seed of each in the zone of the 

 other, and in growing K, purpurea under controlled mesophytic conditions, 

 'and K. Candida under similarly measured xerophytic conditions in the plant- 

 house. 



While the polyphyletic origin of species is in a fair way to be decided by 

 experiment, it receives support from several well-known phenomena. The 

 striking similarity in the plant body of families taxonomically so distinct as 

 the Cactaceac, Stapeliaceae, and Eiiphorhiaceac, or Cyperaceae and Jiui- 

 'caceac, indicates that a vegetation form may be polyphyletic. On the other 

 hand, the local appearance of zygomorphy, of symphysis, and of aphanisis 

 in the floral types of phylogenetically distinct families is a proof of the 

 operation of convergence in reproductive characters. To be sure, the con- 

 vergence is never so great as to produce more than superficial similarity, 

 but this is because the groups are markedly different in so many fundamental 

 characters. The same tendency in closely related species would easily result 



'The Origin of Species, ISG. 1859. 



