1S99] 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 

 Echinoderrn-larvae were found out in the Atlantic, and three of these 

 were in the Sargasso Sea. 1 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 pcllucidus, P. drummondi, and 

 Astcrias mucllcri. Some species of the Mollusca too are absent from 

 Greenland, e.g. Beta trcvclyana. 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 fremdosa, 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 Bohustiln, 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 (Cribrclla, Astcrias muelleri, Crossaster 

 pcqywsus), 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 in a 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. Lonnberg. 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. 



