JEFFREY: EVOLUTION BY HYBRIDIZATION 305 
agency for the formation of species. It appears moreover logically 
impossible to regard hybridization as the universal and sole cause of 
the appearance of new species, as has been,recently maintained by 
Lotsy in his Evolution by Means of Hybridization, since the original 
species must have come into existence by some other means than by 
hybridization. The adaptation of the floral structures of the Angio- 
sperms to cross fertilization, emphasized many years ago by the 
Austrian botanist Kerner is doubtless of significance in connection 
with the ever-increasing volume of evidence for the wide occurrence 
of natural hybrids in this large and successful group of seed-plants 
which have to so notable a degree furnished the facts for the existing 
general biological theories. 
DESCRIPTION OF PLATE V 
Fic. 1. Pollen of Zannichellia palustris, showing great uniformity in a species 
unable to hybridize. X 400. 
Fic. 2. Pollen of Potamogeton diversifolius, showing great diversity of size and 
development in pollen of a species subject to hyridization. X 400. 
Fic. 3. Pollen of Rubus odoratus, a species which flowers late and consequently 
is not subject to crossing. X 400. 
Fic. 4. Pollen of Rubus villosus, a species flowering with a number of others 
and consequently subject to hybridization. X 400. 
Fic. 5. Pollen of Ranunculus rhomboideus, showing uniformity in a species 
exempt from crossing by early date of flowering. XX 4oo. 
Fic. 6. Pollen of Ranunculus acris, a species flowering at the same time as a 
number of others and consequently exposed to hybridization. X 400. 
