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688 THE AMERICAN NATURALIST. [Vow XXXIIL. 
Phalaris, seem to agree in speaking of P. sessilis, P. digraph- 
idis, and P. phalaridis as distinct species, although Plowright 
considered P. paridis to be distinct from P. digraphidis, 
whereas Magnus considered the two to be what he calls adapt- 
ive races (Gewohnheitsracen) of the same species. Magnus 
speaks of the three species as biological species, which he dis- 
tinguishes from adaptive races, the latter including forms in 
which, although the æcidium may be produced on different 
hosts, it does not appear to be so frequent or so well developed 
on some hosts as on others, showing in the one case that the 
adaptation is more complete than in the other. Klebahn, 
although admitting that it is not of real importance whether 
one regards such forms as the Pucciniæ on Phalaris as species 
or races, nevertheless states that he sees no reason why they 
should not be considered to be genuine species rather than 
races. 
Another instance in point is the group of æcidia generally 
known as species of Peridermium, which infest species of Pinus. 
It had for some years been recognized that the æcidial stage 
of the corticolous form of Peridermium pini was not the same 
as that of the form on the leaves, but in recent years the sub- 
division has been carried much farther, owing to cultures made 
by Klebahn, Edouard Fischer, Rostrup, and others. The 
former has distinguished at least seven species of Peridermium 
on Pinus sylvestris alone, whose uredo and teleutospores are to 
be found in the species of Coleosporium, which grow upon dif- 
ferent genera of Compositæ, Scrophulariaceæ, and Campanu- 
laceæ. Although Klebahn is inclined to see minor differences 
in the shape and markings of the æcidial spores of some of the 
species, it must be admitted that the differences in some cases 
are so slight, both in the case of the æcidial spores and the 
corresponding teleutospores, that, were it not that cultures show 
the connection between the form on one host with that on 
another to the exclusion of other hosts, it is hardly likely that 
many botanists would consider them as distinct species. 
The most suggestive Uredinaceæ for our present purpose are 
the different species of Puccinia which attack grains and other 
grasses, for a knowledge of which we are indebted to the 
