2 SOURCES OF A HISTORY OF PLANTS. 



prevail for eight mouths instead of seven, and where the season for vegetation 

 is confined to four months? They would succumb to the inhospitality of the 

 climate; and it follows that a limit to the distribution of the species in question 

 would be attained at a line connecting all places which possess a climate of equal 

 rigour. This does not pi-eclude the possibility of other causes constituting a barrier 

 to the distribution of the same species in other directions. Peculiarities of soil, for 

 instance, may prevent the naturalization of a plant; or, its spread may be baffled 

 by the opposition of plants already long settled in the place invaded; or any other 

 like impediment may operate as a check. Facts of this kind, being brought to 

 light by the comparison of diflerent Floras, led to detailed research into the means 

 of reproduction and distribution in plants, to a study of the many contrivances 

 for their propagation, and of the nature of the equipments which enable the 

 descendants of a stock to enlarge the area where it grows. 



Side by side with these investigations into the history of individual kinds of 

 plants, there was developed a special department of research with the view of 

 determining the actually-existing boundary-lines of disti'ibution — the so-called 

 lines of vegetation — of particular species, and of ascertaining all the conditions 

 of soil and climate afTecting plant-life which prevail along these lines, so as to take 

 into consideration all the possible causes of limits to distribution. The range of 

 observations was likewise extended to displacements of the lines of vegetation, to 

 the advance of particular species in one direction or another, and the suppression 

 and annihilation of others within historic times; thus a chronicle of plant migration 

 was started. 



The unlooked-for discovery of the multitude of plants which flourished upon 

 the earth ages ago, and have been preserved as fossils, led to a further comparison 

 of forms — viz. of those now living with those that have perished. There was no 

 evading the idea tliat existing species are derived from others now extinct; on the 

 contrary it proved so attractive that it was followed up with the greatest interest 

 and zeal. Then tljese inquiries into the parentage of species naturally led further 

 to the whole problem of their origin — in short, to a study of the history of species. 



The range of vision continued to become yet wider. It is impossible that the 

 dwarf willows and birches found living in Greenland at the present day should be 

 descendants of the maples and beeches which grew there in the Tertiary Pei'iod, or 

 that the alders or pines now flourishing on the soil above the beds of bituminous 

 coal at Haring in Tyrol should have sprung from the ProteaeejB and Myrtacea3 

 which formerly covered the same ground, as we learn from the fossil remains found 

 there. Local changes must have taken place, and the various floras must have 

 undergone a process of expatriation on a large scale not unlike that of men at the 

 time of the migration of tribes. New realms were then occupied by those floras 

 in a manner corresponding to the formation of states by the struggling and ming- 

 ling races and nations of mankind. The knowledge of tlie fact tliat a plant's form 

 depends at the present day upon soil and climate entitles us, moreover, to infer that 

 a similar connection existed in past times between the forms of plants and their 



