March, 1902.] 



KNOWLEDGE. 



51 



the lake, it is a qiipstion whether thcv would reaeh auy 

 other lake. The Bur-reed, so coiiimon in ponds and 

 ditches, reijuires less deep and open water than the Pond- 

 weeds just mentioned ; it is one of the plants of the fringe 

 that separates the aerial from the subaqnatic tlora. Its 

 seeds are likewise comparatively heavy, and not possessed 

 of any special means of dispersal, yet it is a more widely- 

 distributed plant than its companion the Reed-mace, 

 which is furnished with innumerable liijht parachute-fruits, 

 which it disperses annually over the country in tens of 

 thousands, by the aid of the wind. Tiie Yellow Iris has 

 very heavy seeds, which cannot easily attain wide dispersal 

 exce])t by running water. But as a damp meadow provides 

 a sufliciently wet snbstr.atum for the j)lant's growth, it has 

 a wi'ler ranire than species which need standing water. 

 Its terrestrial liabitat, too, brings it into contact with the 

 larger animals, wliicli in many ways help to scatter seed 

 of all sorts As to the water-plants before mentioned, 

 ai|Uatic birds perhaps .assist dispersal more than any other 

 agoncv. A great variety of seels is mixed with the mud 

 on the liauks and in the beds of rivers and lakes, and many 

 have been observed adhering with mud to the feet of 

 wading birds. Other seeils have hard coats, which allow 

 tlipui to pass without injury throu;jh tiie alimentary canal 

 of birds — digestion in birds being extremely rapid. That 

 the seeds of the marsh and water plants ilo succeed in 

 crossing the '"deserts "of dry soil that hem them in on 

 every hand is sufliciently evident; not a lake or pond but 

 has its subacpiatic association of Pondweeds, Water- 

 8larworts, Water-Milfoils, Stoneworts, and so on, and its 

 marginal fringe of Bur-reeds, Bulrushes, Reed-maces, and 

 aquatii' sedges and grasses. The " islands " of the opposite 

 kind — v<'rv dry S)>ots — have a more local development. 

 Tiiey consist of sandy and gravelly wastes, and of rocks. 

 As j>ointed out in a previous article, some of the rook 

 plants have taken ailvantage of the wall-building pro- 

 clivities of msmkind to widely extend their domain; and 

 mention has been also made of the fact that certain other 

 xerophytes have followed the narrow strip of tenalde 

 ground supplied by the liallasting of railways right across 

 an inhosi)itable clay-covered country. While the lighter 

 soils and drier climate of the south and east of England 

 permit of the growtli of many such ])Iants, their numbers 

 decrease in the more rainy districts, and in Scotland and 

 Ireland we find the xerophytes aggregated on the sands of 

 the sea-coast. 



Wlien we come to examine the distribution of individual 

 species, we find that while as regards hnbifat they con- 

 sistently seek a due amount of water and a suitable soil 

 and situation, yet their distribution is full of anomalies. 

 What I mean is that while the occurrence of a certain 

 plant points to the presence of certain conditions, the 

 presence of certain conditions does not by any means 

 necessarily involve the occurrence of the plant. This is 

 where the historical element comes in. The flora as we 

 find it now is the result of a long and eventful past. 

 Species have arrived and departed, have advanced and 

 retreated, in obedience to overmastering changes in climate 

 and in the distribution of sea and land. That the bulk of 

 our present flora migrated hither over a continuous land- 

 surface atthedoseof the Glacial Epoch, from those southern 

 parts of Europe which, though mucli colder than at present, 

 were then free from ice, and aide to support a Cool-temperiite 

 flora, we may safely assume. In colonising the country, 

 the more plastic species spread over the land from end to 

 end. The hardier plants followed the retreating ice 

 northwards, and found a congenial home in Scotland, and 

 noi'thern England and Ireland. The sun-loving plants — 

 perhaps for this very reason the latest comers — settled 

 down in the warm and drv eastern counties of England. 



The remnants of the pre-existing flora, composed largely of 

 Arctic-Alpine plants, retired in great part to the moun- 

 tains. Some strange conditions ajijiear to have existed 

 along the old western sea-board, whiidi allcjwed traces of 

 the ancient pre-Glacial flora of southern jilants to survive 

 in Ireland and the south-west of England. 



But even when the heterogeneous elements of the 

 vegetation had settled down in accordance with the 

 prevailing (Conditions, there was no pause in the over, 

 changing order of things. 



" Turn, turn, my wlioel ; all things must ehango 

 To something new, to something strange, 

 Nothing that is can pause or stay." 



Some plants extended their domain ; otliers sank in the 

 grim struggle for existence. The older forms, presumably 

 less suited to the new conditions, may well have found 

 life difficult. Now, when a plant is increasing its area, or 

 is at the high-water mark of its jirosperity, we may expect 

 to find its range continuous over the area which is suitable 

 to its growth, like a pool of water which is enlarging. 

 But if a plant's dominion is on the wane, and its area 

 shrinking, it will retreat irregularly, clinging to its 

 favourite habitats long after it has been swept away 

 from the .adjoining areas ; its distribution will become 

 discontinuous, like that of a pool of water which is drying 

 up, and leaves for a while isolated wet patches in the spots 

 best able to retain water. Thus it is that our arctic 

 plants have shrunk back to the mountain-tops, where they 

 now exist often in a perfectly isolated condition, and cut 

 oif from all communication witli tlieir former companions, 

 imprisoned on other peaks. To quote equally conspicuous 

 instances of iiirreasing species is not easy, as the only 

 l>lants of the kind which one can see increasing are weeds 

 introduced by man, and consequently not subject to 

 natural conditions ; but a glance at the map in the 

 January number of Knowledge, in which I illustrated 

 the recent spread of three introduced railway colonists, 

 will show that even each of these has a continuous range, 

 and the appearance of having originated from a single 

 centre of dispersal. Let us look more closely into some 

 of these ca.ses of discontinuous distribution. The most 

 conspicuous instances belong to plants of distinctly 

 northern or southern type. A number of species of the 

 Arctic-Alpine group, left stranded on our mountain-tops 

 as the cold of the Glacial Epoch retreated, are now repre- 

 sented by solitary plant-islets amid the sea of more 

 southern forms, for instance : Arabis alpina (one spot in 

 Skye) ; Arenaria ciliata (King's Mountain, co. Sligo) ; 

 A. ulujinosa (by one rill in Teesdale) ; Saa-ifraga 

 cernua (Ben Lawers) ; Mvlgedivm. alpimtm (Forfar and 

 Aberdeen) ; Phyllodoce cnerulea (Sow of Athol) ; Gentiana, 

 nivalis (Breadalbane and Clova) ; Pinguicula alpina (Skye 

 and Ross). These are all plants of the high north, or of 

 the alpine portions of Europe. 



The southern plants which form islands are often less 

 rigidly restricted in their range, since they are low-level 

 dwellers, and have better opportunity of spreading 

 liorizontally. Enphorhia hiherna and Saxifraga umbrosa, 

 for instance, have a wide range along the western and 

 southern coasts of Ireland, the latter showing its flexibility 

 by ascending the mountains into the haunt of the alpines. 

 Pinguicula qrandiflora grows in profusion over great j)art 

 of Cork and Kerry, an area of several thousand square 

 miles, ascending like the last to a considerable elevation. 

 All of these are southern forms. The most striking 

 instances of these jdant-islands, however, are those formed 

 by species which have a wide range over the European 

 continent, and might, therefore, be expected to find, so far 

 as the all-important factor of climate is concerned, a 

 suitable home in many parts of our islands. Yet remark- 



