JEFFREY: EVOLUTION BY HYBRIDIZATION 303 
tation. In monotypic genera such as Ginkgo, Liriodendron, Calla, 
Spathyema, etc., the pollen grains under normal physiological condi- 
tions of development are all alike and perfect. In Fig. 1, Plate V, 
is shown the pollen of Zannichellia palustris, a species isolated in our 
northern North American flora. It is clear that the grains are strik- 
ingly uniform and are all well developed. For comparison with the 
genus just mentioned which has very few species and consequently 
cannot be considered as highly variable, let us take the common 
pondweed Potamogeton, of which there are very many species. Fig. 2 
shows the situation in the large genus just mentioned. The cells are 
not by any means all perfectly developed and are conspicuously char- 
acterized by extreme variations in size. It might be maintained on 
this basis of the illustrations furnished in Figs. 1 and 2 alone that 
variability in size of pollen grains is associated with the multiplication 
of species or in other words with the phenomenon of mutation. 
Against this view in the forms under discussion may properly be urged 
the fact that many natural hybrids between species of Potamogeton 
are known which manifest the usual morphological features of such 
forms. 
A clearer elucidation of the situation is furnished by the conditions 
in large genera, where a number of the species coincide both in geo- 
graphical distribution and in the time of flowering. As a first illus- 
tration in this connection may be taken the genus Rubus, which has 
recently been investigated by Dr. Hoar. Fig. 4 shows the condition 
of the contents of the anther in R. villosus. Clearly the pollen varies 
greatly in size and perfection of development. A similar condition 
has been described by the author just cited in a large number of the 
species of Rubus. The general situation might be interpreted in view 
of the very numerous and at the same time very variable species of 
Rubus as an argument for the correlation of mutation and _ pollen 
sterility. When however the facts in species of the genus, which are 
in some manner isolated, are examined quite a different light is thrown 
on the subject. Fig. 3, Plate V, illustrates the pollen of R. odoratus, 
the flowering raspberry, which opens its blossoms at a considerably 
later period than the mass of the species of the genus. Care has been 
taken to include as large a number as possible of the grains in the field 
-of view. It is obvious that the variation in size and frequent defective 
development of unisolated species of Rubus, are conspicuous by their 
absence. If irregularities in the development of the contents of the 
anthers were a feature correlated with mutation in the genus Rubus 
then we ought to find it equally present in isolated and unisolated 
species. Since that is not the case, the natural inference is that the 
sterility present in the pollen of species subject to hybrid contamina- 
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