60 MR. A. W. BENNETT ON THE 
Phæosporeæ through the Dictyotew, which agree with the Flo- 
ride: in the absence of spontaneous motility in both the sexual 
and non-sexual reproductive elements. But it seems improbable 
that modes of reproduction so closely resembling one another as 
those in the Coleochstaces and the Florides should have arisen 
independently. In the higher families of the Floride® we find 
also the highest development of the organs of assimilation and con- 
duction among Thallophytes. If the more complicated types of 
Floride: are derived by direct descent from the Coleochstace®, 
it follows that the less highly developed families must be regarded 
as retrogressions from the parent type; and this theory, I ven- 
ture to think, offers the most probable explanation of the true 
position of some aberrant forms. In the Nemaliee and Squa- 
marie@ the degeneration is exhibited solely in the less perfect 
development of their thallus, the mode of reproduction being still 
of the normal character. In the Lemaneacee this is accom- 
panied also by a simpler structure of the sexual organs. But here, 
as wellas in Batrachospermum, we have the first rudimentary 
appearance (except among Fungi) of the phenomenon known as 
“ alternation of generations," which attained its acme in the 
Vaseular Cryptogams, and which may possibly indicate the 
genesis of the Musciner. In the Bangiacee or Porphyracex we 
have a reduction of the thallus to a simple filament or plate of 
cells, accompanied by a rudimentary development of both oogones 
and trichogynes, and a limited reversion to motility in the tetra- 
spores. For reasons stated above, it seems to me that we must 
regard the Bangiacee as exhibiting retrogression from the more 
complicated Floridee rather than as the lowest member of an 
ascending series; and, if this view is adopted, a place will be 
found for the Ulvacee as derivatives from the Bangiacee by 
further retrogression, displayed in the entire suppression of 
oogones and antherids, and reversion to an ealier mode of 
reproduction—the conjugation of zoogametes. In a purely sexual 
system of classification the Ulvacee must, of course, be placed 
among the Zygophycee ; but their vegetative structure differs 
widely from all the other members of that group, while the affi- 
nity of Ulva to Porphyra can scarcely be doubted. 
This completes the attempt at tracing the affinities of the 
various families of Alge. The Characes, though included by 
some writers under Algs, are best dissociated from them in con- 
sequence of their distinct cormophytic structure. Their genesis 
