1890.] The Distribution of Plants. 823 
In 1856 and '57 Dr. Gray published in the American Journal 
of Science a continued article on the Statistics of the Flora of the 
Northern United States,’ in which facts in line with those already 
indicated were brought out at much greater length, statistical 
comparisons being made between the numbers of orders, genera, 
and species indigenous to the Northern United States and those 
of Europe and Eastern Asia respectively; the close relationship 
of the floras of the two great continents again being brought out 
in a still more striking manner. His remarks on the theoretical 
bearing of these facts are of special interest from having appeared 
some little time before the “ Origin of Species" Dr. Gray says: 
“As the discussion of this most difficult problem proceeds, the 
two antagonistic positions only appear to be tenable. 
The first theory is based upon the natural idea of species as con- 
sisting of kindred individuals descended from a common stock 
which, whether demonstrable or not as a fact, gives us a clear 
and distinct conception of species, and the only one we possess, 
The second theory, being incompatible with this conception, 
leaves species no objective basis in nature and seems to make 
even the ground of their limitation a matter of individual opin- 
ion.” 
Here was the essential conception of the real nature of species, 
—a conception that became more fixed as his studies continued, 
and was expressed more at length in a memoir presented to the 
American Academy in 1858-'59,° in which Dr. Gray says : “ The 
natural supposition is that individuals of the same kind are de- 
scendants from a common stock, or have spread from a common 
center ; and the progress of investigation, instead of eliminating 
this preconception from the minds of botanists, has rather con- 
firmed it. 
Without attempting to condense or reproduce further the sub- 
stance of these earlier papers, it is enough to say that in them 
had already been clearly formulated two essential principles, viz., 
. the genetic relationship of plants of the same and “ representa- 
tive” species, and repeated migrations under changed climatic 
1 Am. Jour. Sci., 2d Ser., Vol. XXII. (1856), and Vol. XXIII. (1857). 
. * Memoirs Am. Acad., New Ser., Vol. VI. 
