( vii ) 
in the highest tribe only by a well-defined intermediate one. In one of 
these (typically colourless) the originally simple spore, passing through a 
series of modifications, always in one direction, and tending constantly 
to elongation, affords at length the acicular type. To this is opposed 
(most frequently but not exclusively in the lower tribes, and even possibly 
anticipated by the polar-bilocular sub-type in Parmeliacei) a second 
(typically coloured) in which the simple spore, completing another series 
of changes, tending rather to distention, and to division in more than one 
direction, exhibits finally the muriform type. Differences such as these 
appear certainly to be significant; and to suggest a possible correlation 
with others, which shall leave no doubt that these types’ require marked 
expression in the system. Nor is such expression questioned in the case 
of the best-developed, foliaceous groups. Nobody now hesitates to dis- 
tinguish Physcia and Pyzine from Parmelia, or Solorina from Peltigera ; 
and the argument from such foliaceous to the analogous crustaceous 
genera is impeded perhaps by nothing beside the thalline inferiority of 
the latter. But it is seen at once that the case is not the same with the 
successive steps in the process of differentiation of these types; and the 
value of such gradal (bilocular, quadrilocular, plurilocular) distinctions 
should be clearly inferior. Species which exhibit the ultimate condition 
of their spore-type, as here taken, exhibit also, ideally at least, the whole 
of the preceding process of evolution. This is still better observed in 
larger, natural groups, as (exc. excip.) Biatora vernalis, Fr. L. E., express- 
ing, with general congruity of structure, the whole history of the colour- 
less spore. And the step is not a long one from such groups to natural 
genera; to the assumption that gradal differences of the same type of 
spore, displayed by species, or clusters of species, within the circuit of 
what is otherwise a natural genus, shall be an insufficient ground for the 
sundering of such genus. The consideration of the numerous, sometimes 
sufficiently significant instances, in which nature appears to point in this 
direction, will be attempted further on. Suffice it here to say that, 
according to these views, Parmelia proper, Ach., will fall into Thelos- 
chistes, Parmetia, and Physcia; and Lecanora into Placodium (DC.) 
Naeg. & Hepp, Lecanora, and Rinodina. Excluding the biatorine forms 
of Placodium from the Lecideei, the latter will have no examples of the 
polar-bilocular sub-type; but Heterothecium, corresponding to Physcia 
and Rinodina, will be distinguishable from Biatora ; and Buellia similarly 
from Lecidea. And the whole class may be conceived, as in like manner 
