KINDS OF INVASION 77 



well-known phenomena. The striking similarity in the 

 plant body of families taxonomically so distinct as the 

 Gaxitaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and 

 Juncaceae, indicates that a vegetation form may be polyphy- 

 letic. On the other hand, the local appearance of zygo- 

 morphy, of symohysis, and of aphanisis in the floral types of 

 phylogenetioally distinct families is a proof of the operation 

 of convergence in reproductive characters. To be sure, the 

 convergence 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 

 in identity. As in the case of polygenesis, the relatively 

 small number of typically distinct habitats makes it clear 

 that two different species of wide distribution, bearing to 

 each jOther the relations of xerophyte to mesophyte, of 

 hydrophyte to mesophyte, or of poophyte to hylophyte, 

 might often find themselves in reciprocal situations, with the 

 resalt that they would give rise to the same new form. The 

 final proof of the polyphylesis of species is afforded by the 

 experiments of De Vries in mutation. De Vries found that 

 Oenotheia nanella arose from 0. Lamar ckiana, 0. laevifolia, and 

 0. scintillans: Oenothera scintillans arose from 0. lata and 

 0. LamarcMana; Oenothera rubrinervis from 0. Lamarckiana, 

 0. laevifolia, 0. lata, 0. oblonga, 0. nanella and 0. scintillans, 

 etc. Whatever may be the rank assigned to these mutations, 

 whether form, variety or species, there can be no question of 

 their polyphyletic origin, nor, in consequence of the con- 

 nection of mutations with variations through such incon- 

 stant forms as O. scintillans, 0. elliptica and 0. suMinearis, of 

 the possibility of polyphylesis in any two distinct though 

 related species or genera. 



KINDS OF INVASION. 



With respect to the frequency of migration, we may dis- 

 tinguish invasion as continuous {invasio continua), or 

 intermittent {invasio infrequens). Continuous invasion, 

 which is indeed usually mutual, occurs between contiguous 

 formations of more or less similar character, in which there 



