FOREST GEOGRAPHY AND ARCHZOLOGY. 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. Itis 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 
