The Evolution of the Higher Uredineez. 103 
intercalation is, as was to be expected, a lengthening of the 
sporophytic generation. 
When the sporophytic generation was still further lengthened, 
by the introduction of uredospores, it is certain that, of those which 
had all the three spore-forms, the autoecious species would be first 
evolved. There are then three possible hypotheses by which the 
origin of hetercecism could be explained. The first is due to Dietel, 
(7) and agrees with the one suggested at the beginning of this 
article. It is approved by Klebahn (16). The second is Dietel’s 
later alteration of his hypothesis (8) : he supposes that the primitive 
form of any group of species possessed only teleutospores, and was 
confined to one host: the present state of things arose (1) by 
intercalation, (2) by an accompanying mutation signified by the 
migrating of the aecidiospores on germination to a new host. This 
is also incorporated in the hypothesis here advocated, although in 
a sense not conceived by Dietel. The third is due to Fischer; (11) 
he supposes that the primitive form was plurivorous and possessed 
all spore-forms: then a further development took place by which 
some of the evolved species restricted certain of their spore-forms 
to hosts of particular families and the others to hosts of other 
families {i.e., became hetercecious), while at the same time by 
suppression of some of the spore-forms Micropuccini-as and 
Brachypucciniae arose. Thus, to return to the case of Puccinia 
fusca and its allies, the primitive form would be supposed to possess 
all spore-forms and to occur with all of them indiscriminately on 
Anemoneae and Pruneae. While certain abnormal cases might, 
perhaps, be more easily explained on this hypothesis, it labours 
under several difficulties. In the first place, it affords no explan¬ 
ation of Dietel’s “ Rule,” and in the second there does not seem to 
exist a species of Uredineae which is plurivorous in the required 
sense, i.e., has all its spore-forms equally and indiscriminately on 
hosts belonging to diverse families. While the other hypotheses 
do not explain all the known cases, e.g., that of Puccinia Isiacce 1 
or Uromyces Scirpi, they can, at any rate, as shown above, present 
actual instances of existing species which satisfy the theoretical 
requirements. 
' Puccinia Isiaccc has its teleutospores, so far as is known, on Phragtnites 
alone, but its accidia have been produced by Tranzschel on twenty-one plants 
belonging to nine widely different families, ranging from Crucifera: to 
Scrophulariaceae. Manufactured instances, however, are not much to the 
point. 
