FOREST GEOGRAPHY AND ARCHAEOLOGY. 225 



That is, in these days it is taken for granted that individ- 

 uals of the same species, or with a certain likeness through- 

 out, had a single birthplace, and are descended from the same 

 stock, no matter how widely separated they may have been 

 either in space or time, or both. The contrary supposition 

 may be made, and was seriously entertained by some not very 

 long ago. It is even supposable that plants and animals orig- 

 inated where they now are, or where their remains are found. 

 But this is not science ; in other words it is not conformable 

 to what we now know, and is an assertion that scientific ex- 

 planation is not to be sought. 



Furthermore, when species of the same genus are not found 

 almost everywhere, they are usually grouped in one region, 

 as are the Hickories in the Atlantic States, the Asters and 

 Goldenrods in North America and prevailingly on the Atlan- 

 tic side, the Heaths in western Europe and Africa. From 

 this we are led to the inference that all species closely related 

 to each other have had a common birthplace and origin. So 

 that, when we find individuals of a species or of a group 

 widely out of the range of their fellows, we wonder how they 

 got there. When we find the same species all round the 

 hemisphere, we ask how this dispersion came to pass. 



Now, a very considerable number of species of herbs and 

 shrubs, and a few trees, of the temperate zone are found all 

 round the northern hemisphere ; many others are found part 

 way round, — some in Europe and eastern Asia ; some in 

 Europe and our Atlantic States ; many, as I have said, in the 

 Atlantic States and eastern Asia ; fewer (which is curious) 

 common to the Pacific States and eastern Asia, nearer though 

 these countries be. 



We may set it down as useless to try to account for this dis- 

 tribution by causes now in operation and opportunities now 

 afforded, i. e., for distribution across oceans by winds and cur- 

 rents, and birds. These means play their part in dispersion 

 from place to place, by step after step, but not from continent 

 to continent, except for few things and in a subordinate way. 



Fortunately we are not obliged to have recourse to over- 

 strained suppositions of what might possibly have occurred 



