Subordinate and Herbaceous Flora 45 



sion to this part of the world, the general sequence of events is so far 

 outlined, and it should be sufficiently clear that : 



(1) Every aquatic of the open stream once grew on the bank. 1 



(2) Every bank-aquatic was once a plant of damp woodland under- 

 growth. 2 



(3) Every plant of swamp-woodland was once in normal woodland as 

 a herbaceous perennial. 



(4) Every herbaceous perennial was once an underwood shrub. 



(5) Every underwood shrub was once a tree of high-forest. 



Each stage in such progression calls for further detailed analysis ; 

 but the present object is to show that the story is unified, and hangs 

 together. A local flora is not a collection of disjointed units, or phases of 

 special creation, but one progressive whole, in which each part has its 

 proper value and status. This is what is implied by the evolutionary 

 and ecological standpoint. 



The advanced submerged obligate aquatic is perhaps the type furthest 

 removed from the primary forest-tree ; and just as the tree-type required 

 to invent and specialize its arboreal factors from the horizon of a trans- 

 migrant seaweed, so these factors are gradually lost in the herbaceous 

 perennial on regression to water as a rhizomatous form, or are retained as 

 vestigia, to be last discerned in the floral shoots and details of the floral 

 mechanism of reproduction. It is necessary to have some working hypo- 

 thesis in tracing such a progression, and the foregoing assumptions are 

 based on the causal factors of light and water-supply as bearing on the 

 nutrition of the individual, the loss of primary arboreal factors being 

 compensated by precocity of flowering and fruiting, with increasing 

 specialization in the mechanisms of cross-pollination and seed-dispersal, 

 as also of increased vitality in perennation. Conversely, while it appears 

 possible to trace the progression of the obligate aquatic, by the loss of all 

 arboreal factors, there is little evidence that such a series will ever work 

 backwards all the way, or that the full type of a forest-tree has been ever 

 derived from such aquatic' vegetation. The story of the Flowering Plant 

 (or Angiosperm) is restricted to primary forest-forms, the origin of which 

 from necessarily smaller types of transmigrant vegetation appears at present 

 hopelessly beyond recall ; though the existence of primary land-plants in 

 the herbaceous form may be indicated at the older horizons of Bryophyta 

 and possibly Pteridophyta. 



It seems a far cry from the Duckweed of a standing pond to a timber- 

 tree of high tropical forest ; since the time required for such a transition is 

 beyond our perception, once it is granted that there is no reason to believe 

 that the rate of progressive evolution has been ever greater than it is at the 

 present day. A period of a thousand years makes no appreciable difference 

 either in climatic conditions, or on the indigenous flora. The December 

 frost of 1142 was recorded as a phenomenon, and was no more intense than 

 that of March 1895. The vegetation was essentially the same in woodland 

 and any valley-pastures in Roman and Neolithic times ; and it has been 

 wholly renewed since the last maximum of Glacial cold. The time since 

 the post-glacial river-terraces were begun must be reckoned in hundreds of 

 thousands of years ; when it is noted that the greater depth of the river- 

 valley was cut down apparently in milder epochs, with no marked snowfall, 

 much as at the present day, with slow erosion and deposition of alluvium. 



1 Since all (with the exception of Wolffia) retain roots as essentially soil-organs. 



2 Rhizomes of many plants follow the water down the bank, to 3-6 ft. trails, extending into the 

 water, or floating. Cf. Epilobium hirsutum, Potentilla rcptans, Lycopus europaeus, Polygonum 

 amphibium, Agrostis stolonifera, in widely related families. 



