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 



Fig. I. Pollen of Zannichelka palustrts, showing great uniformity in a species 

 unable to hybridize. X 400. 



Fig. 2. Pollen of Potamogeton diver sifolius, showing great diversity of size and 

 development in pollen of a species subject to hyridization. X 400. 



Fig. 3. Pollen of Rubus odoraltis, a species which flowers late and consequently 

 is not subject to crossing. X 400. 



Fig. 4. Pollen of Rubus vlUosiis, a species flowering with a number of others 

 and consequently subject to hybridization. X 400. 



Fig. 5. Pollen of Ranunculus rliomboideus, showing uniformity in a species 

 exempt from crossing by early date of flowering. X 400. 



Fig. 6. Pollen of Ranunculus acris, a species flowering at the same time as a 

 number of others and consequently exposed to hybridization. X 400. 



