1899] THE FAUNA OF THE SOUND 271 
the coasts, and do not belong to the true plankton of the high seas. 
The Hensen Plankton - expedition only once got as many as three 
Echinoderm-larvae at any distance from land. Only five species of 
Echinoderm-larvae were found out in the Atlantic, and three of these 
were in the Sargasso Sea.’ Again, this first hypothesis does not 
explain why it is that these Arctic forms should be found in Oresund and 
not in other places, such as the northern Kattegat, where the oppor- 
tunities for their development seem equally favourable. Moreover, 
many of the forms in question stretch north-eastwards along Finmark 
to Spitzbergen and the Kara Sea, but are not known from the coasts of 
Greenland: such are Phyllophorus pellucidus, P. drummondi, and 
Asterias muellert. Some species of the Mollusca too are absent from 
Greenland, e.g. Bela trevelyana. But from Spitzbergen and the sur- 
rounding seas no current leads to the Kattegat. Then, too, if the 
first hypothesis were true, we should expect to find many other Green- 
land species, which, as it happens, are absent not only from the Sound 
but also from the Kattegat and Skagerack. Of twenty-nine species of 
Echinoderms found in Greenland, only eight occur in the Kattegat and 
the Sound. If some can cross, why not others? Take the case of 
Cucumaria frondosa, a holothurian common in Greenland waters, and 
with so wide a distribution that it stretches down America as. far as 
Massachusetts, and down Europe from the North Cape to the English 
Channel. Yet it is absent from Bohustiin, the Kattegat, the Sound, and 
Helgoland. This is a strong argument against the Greenland current 
theory. <A still more forcible objection is furnished by the fact that 
some of the starfish in question (Cribrella, Asterias muellert, Crossaster 
papposus), and perhaps other of the Echinoderms, have no pelagic larval 
stage at all. 
It is clear that the first hypothesis fails us at many points. We 
have then to consider the second, and to inquire how long and where- 
fore these forms have remained in a district so isolated from the rest of 
their area of distribution. Two main groups of conditions determine 
the persistence of an animal ina given locality. One group includes 
the external chemical and physical conditions ; the other, the relations 
of the organic world. The Sound, therefore, must afford conditions 
suited to the existence of Arctic animals, and at the same time less suited 
to the more southern forms with which they have to struggle. Arctic 
forms are accustomed to a low temperature, and also to great changes 
in the.salinity of the water consequent on the melting of the ice. 
1 Two considerations seem to be overlooked by Dr. Lénnberg. First, the fact that a 
species can develop rapidly does not prove that it must. Experiment has shown that 
development may be greatly retarded by varying the conditions, and, for all we know, 
the necessary stimulus to complete development may be wanting in the current from Green- 
land so long as it is far from land. Secondly, as Alexander Agassiz, for one, has insisted, 
Echinoderms can be transported in other than the larval state ; the young sea-urchin itself 
can be floated along. Especially is this the case when drift-wood or floating sea-weed comes 
to their aid. 
