234 , THE FORMATION 



in indentity. 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 other the relations of xerophyte to mesophyte, 

 of hydrophyte to mesophyte, or of poophyte to hylophyte, might often find 

 themselves in reciprocal situations, with the result that they would give rise 

 to the same new form. The final proof of the polyphylesis of species is 

 afforded by the experiments of DeVries in mutation. DeVries found that 

 Oenothera nanella arose from O. Lamarckiana, O. laevifolia, and O. scintil- 

 lans; Oenothera scintillans arose from O. lata and O. Lamarckiana ; Oeno- 

 thera rubrinervis from O. Lamarckiana, O. laevifolia, 0. lata, O. oblonga, 

 O. nanella, and O. 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 connection of 

 mutations with variations through such inconstant forms as O. scintillans, 

 0. clliptica, and O. sublinearis, of the possibility of polyphylesis in any 

 two distinct though related species or genera. 



K/NDS OF INVASION 



282. Continuous and intermittent invasion. With respect to the fre- 

 quency of migration, we may distinguish invasion as continuous, or 

 intermittent. Continuous invasion, which is indeed usually mutual, occurs 

 between contiguous formations of more or less similar character, in which 

 there is an annual movement from one into the other, and at the same time 

 a forward movement through each, resulting from the invaders established 

 the preceding year. By far the greater amount of invasion is of this sort, 

 as many readily be seen from the fact that migration varies inversely as the 

 distance, and ecesis may decrease even more rapidly than the distance 

 increases. The significant feature of continuous invasion is that an outpost 

 may be reinforced every year, thus making probable the establishment of new 

 outposts from this as a center, and the ultimate extension of the species 

 over a wide area. The comparatively short distance and the regular alter- 

 nation of migration and ecesis render invasion of this sort very effective. 

 An excellent illustration of this is seen in transition areas and regions, which . 

 are due directly to continuous and usually to mutual invasion. Intermittent 

 invasion results commonly from distant carriage, though it may occur very 

 rarely between dissimilar adjacent formations, wh(Mi a tcmporar\' swing in 

 the physical factors makes ecesis possible for a time. It is characterized by 

 the fact that the succession of factors which have brought about the in- 

 vasion is more or less accidental and may never recur. Intermittent invasion 

 is relatively rare, and from the small number of disseminules affected, it is 

 of little importance in modifying vegetation quantitatively. On the other 



