300 Bibliographical Notices. 
compared the spores with those from the coal. When gently crushed, 
the identity of the appearance presented by these forms from the 
coal was very striking. The triradiate markings of the latter were 
almost exactly like the flattened three radiating lines which mark 
the upper hemisphere of the macrospores of Jsoétes lacustris. The 
writer therefore concluded that the forms in the coal were from a 
group of plants having affinities with the modern genus Jsoétes, and 
from this Isoétoid character he suggested for them the generic title 
of Isoétovdes pending further investigation. 
BIBLIOGRAPHICAL NOTICES. 
Notes on Natural Selection and the Origin of Species. By Francis 
P. Pascoz, F.L.S. London, 1884. Taylor and Francis. 
Mr. Darwin having outlived the unreasoning rancour of his early 
critics, his works are likely for some time to come to form the text 
for much useful, thoughtful, and no doubt, in many cases, well- 
founded criticism truly so called. Mr. Pascoe’s ‘ Notes’ belong to 
this latter category. He admits that ‘“‘ no naturalist in these days 
doubts that species have arisen by modifications through descent ;” 
and he offers no suggestion as to the fixation of specific characters 
by any means except natural selection. His object is simply to 
point out various classes of difficulties in the way of the acceptance 
of this, which is after all the Darwinian theory, and, in addition to 
apparently endorsing many of Mr. Mivart’s criticisms, to insist on 
the view that all the characters which serve to differentiate species 
are ‘“‘ unimportant except as incipient structures, to which, as yet, 
no advantage can be attached.” This difficulty, and it is a weighty 
one, is, in the present writer’s opinion, partly owing to the unfor- 
tunate prominence given to the indefinite, subjective term “ species ” 
in the title of Mr. Darwin’s great work. It is apparently forgotten 
that a “species” is not so much a series of forms similar iu their 
main positive characters as a series isolated from other series by 
negative characters—in the language of logicians, “ genus et diffe- 
yentia.’” Hence the problem of the origin of species is not one of the 
acquisition of positive characters, but deals with the isolation of 
groups of variations by the extermination of forms intermediate 
between the variations of one geological age and those of another. 
‘It is not the origin of specific characters, but of specific divergence 
or difference. Hence, admitting, as one undoubtedly must, that 
the positive diagnostic characters of species are very generally indif- 
ferent from the point of view of utility, they may yet have well 
become characteristic by the extermination of intermediate stages 
which may very probably have been harmful to the organism. 
No one can deny the existence of a keen struggle for existence, 
and surely it is no “‘assumption ” to state that in this struggle the 
