28 
THE AMERICAN BOTANIST. 
are far smaller. But every one knows how difficult it is to distin- 
guish the common oak from Quercus sessilifora, or the lime tree 
from Tilia grandifoUa. Yet these are forms which by the dis- 
ciples of Linnaeus are recognized as true species. And what 
botanist has not been entangled in the species of Hieraciiim, or 
who is able to recognize at first sight the closely related forms of 
Coclilcria? 
Because of thd dying out of intermediate forms, more ancient 
species may be widely separated. On the other hand, more re- 
cent species, whose ancestors are still alive, may form narrow 
groups because of and wath these surviving ancestors. Good ill- 
ustrations of the latter are yielded by roses, willows and brambles 
as shown by the facility with which the closely related forms can 
be cross-fertilized, as well as by the great trouble the numerous 
bastards cause in determination. Such genera are found every- 
where in the plant kingdom; the gentians of the Alps, for in- 
stance, or the Heliantheinums, which with us seem to be compos- 
ed of fairly distinct types. Everything indicates that in these 
cases the species are of more recent date, and that only through 
the dying out of intermediate forms the differences between the 
remaining ones have attained that degree of distinctness which 
so greatly facilitates the separation of the other groups. 
In this regard the Oenotheras agree exactly with what may be 
observed in nature. Recent forms group themselves around the 
mother form with minute, hardly perceptible gradations. 
Once formed, the new species are as a rule at once constant. 
No series of generations, no selection, no struggle for existence 
are needed. Each time a new form has made its appearance in 
my garden, I have fertilized the flowers with their own pollen 
and have collected and sown the seed separately. The dwarf 
forms produce nothing but dwarfs (0. nanella), the white ones 
nothing but white ones (O. aJbida), the O. gigas nothing but O. 
gigas, the red-nerved ones nothing but corresponding specimens. 
But a single form made an exception. This was the small O. 
scmtillmis, the seeds of which produced but a percentage of sciu- 
fillans plants, but here this inconstancy is and was as much th«^ 
rule as the constancy of the other species. 
As an example I may cite 0. gigas The plant is as tall as 
