XXI 
on the other hand that its importance was by no means so apparent for the distribution of the 
Cirripedia as might be expected, was shown by the fact that whilst many species of Scalpellum 
were found at places with a muddy bottom, several other species were obtained from a bottom 
of hard sand, of coarse sand, or of coral-sand, etc. 
So far as our knowledge goes, we must consider most species of Sca/pellum and Verruca 
as living solitary; as the number of species of these genera (and of Sca/pellum especially) is 
very large, they may furnish valuable evidence for the theories about the influence of isolation 
on the origin of new species, which were brought forward originally by Morirz Wacner! and 
were criticised, and adopted, but only in a much modified form, by WeisMANN’. It is well-known 
that, whilst Wacnrer defended the hypothesis that the formation of a new species took place 
only in consequence of migration and the isolation caused by it, WEISMANN tried to demonstrate 
that not only might a species, while remaining at the same locality, very well divide into two 
or more forms, but also that isolation did not necessarily lead to the development of varieties. 
So migration alone, even if it caused a complete isolation, was inadequate as the sole cause of 
mutation. On the other hand, Weismann agreed that isolation could be one of the factors 
causing the origin of new forms, in the first place, because the isolation prevented the crossing 
with individuals of the same species from the original habitat, and secondly, because it trans- 
ferred the migrant and its descendants to new surroundings. From this point of view, looking 
at so many places at the bottom of the ocean, most of which present special conditions of life, 
depth, bottom, etc., and must indeed be considered as practically isolated from others, we are 
compelled to agree that circumstances there have been favourable to the developing of new 
forms. And then, we certainly cannot wonder that a genus like Sca/pel/wm, which has survived 
from the Cretaceous period down to the present time, being represented in numerous secondary 
and tertiary deposits, and which as a recent genus is found living almost in all the seas and 
oceans of the world, has yielded excellent material for mutation and so for the development 
of new forms or species under the influence of isolation. 
On the same occasion I pointed out that, with respect to the relation of the deep-sea 
forms to the extinct Cirripedes of which fossil remains have been preserved, the “Siboga”’ material 
fully confirmed the conclusions arrived at by the working up of the “Challenger’-Cirripedia. 
So I have not much to add to the views I have expressed already. I wish only to emphasize, 
that, so far as the evidence goes, the different genera show very remarkable differences with 
respect to that relation. It seems, as though we should consider this relation as belonging to 
the properties peculiar to each genus, although we are unable for the present to understand 
why the different genera differ so much in this respect. What I mean by these differences may 
be shown by the following comparison of some of the more important genera. 
Genus Pollicifes, one of the oldest-known genera of the Lepadidae and of the Cirripedia in 
general, is represented in secondary and tertiary strata by numerous species. It 
still exists, but the number of recent species is small, 6 in all, and they occur only 
in warmer temperate and tropical seas, and are all coastal forms. 
1 WaGNER, Moritz, Die Darwin’sche Theorie und das Migrationsgesetz der Organismen. Leipzig, 1868. 
2 WEISMANN, AUG., Ueber den Einfluss der Isolirung auf die Artbildung. Leipzig, 1872. 
