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 Scalpellum and Verruca 

 as living solitary ; as the number of species of these genera (and of Scalpellum 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 Moritz Wagner 1 and 

 were criticised, and adopted, but only in a much modified form, by Weismann 3 . It is well-known 

 that, whilst Wagner 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 Scalpellum, 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"-Cirripeclia. 



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 Pollicipes, 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. 



