4 io The Origin of Species from Mongrels [July, 
with this view the occurrence of polymorphous forms can in 
many cases be explained by adaptation to modifie c ir ^ 
and local circumstances. Thus the development oi the 
manifold forms in the group Viola tricolor, where we hnd the 
most distina forms adapted to distin< 5 t local conditions, can 
be very well comprehended on Darwinian principles, bu 
for the explanation of polymorphism in other groups ot 
plants the principle of variation and selection proves ”jsum- 
cient. As an example we may take the blackberries (black- 
fruited European Ruhus). Herr Focke entered upon e 
study of this group as early as 1857, i.e., prior to the pu 1- 
cation of Darwin’s great work, with the purpose of deteCti g, 
if possible, the process by which species originate. In this 
rich group, although the individual forms chiefly be ong 
well-defined races or species, all stages of the process of e 
formation of species are well represented. The fertility : 
the different races varies as greatly as the structure ot then 
pollen. In development of characters, in independence, or 
connection with kindred forms there occur all imagina e 
differences, so that we cannot conceive of any grade between 
the variety and the “ good species ” which is not to be foun 
among the blackberries. , 
Particular attention must be given to the texture ol the 
pollen in the different forms of blackberry. Some few, such 
as Ruhus ccesius (Linn.), R. ulmifolius (Schott), R. tomentosus 
(Bceck), have a perfectly well-formed pollen. In others we 
find a mixture of the granules,— A e., among sound, pollen- 
grains there is a greater or smaller number of mistormea 
useless particles. Such a degeneration of the pollen may 
be due to the influence of an unfavourable climate or to de- 
fective nutrition. But all the blackberries are characterised 
by their wide distribution, their remarkable constancy, ana 
their relative independence of soil and climate. Whiis 
among the blackberries one and the same race readily adapts 
itself to very different local conditions, we find, on the other 
hand, on one and the same spot of ground a great number 
of different races growing promiscuously. _ It seems, there- 
fore, impossible to assume that the partial failure of the 
pollen is here a consequence of abnormal vital conditions. 
We must rather suppose that, as in numerous other cases, 
the heterogeneous condition of the pollen is connected wi 1 
a hybridity of descent. That the blackberries do in iact 
very frequently produce hybrids is certain. R» ccesius ei- 
tilises all the other species with which it occurs in common, 
and, like various other species, is always accompanied by its 
hybrid progeny. 
