Combination. 



243 



Those who have considered this question seem to have forgotten 

 that also A. E. Verrill has expressed an opinion with reference to 

 transportation by ice. 



In 1883, dredgings were made from S S „Albatross" in the 

 region of the Gulf Stream from off Cape Batteras to Neva Scotia. 

 In the ..Results of the Explorations" (p. 507) Verrill writes: 

 ..In many instances we have also dredged pebbles and small, rounded 

 bowlders of granite and other crystalline rocks from beneath the 

 Gulf Stream in deep water. These, I suppose, have been carried 

 to that region by shore-ice floating off in great quantities from our 

 northern coasts in winter and spring, and melting where the warm 

 Gulf Stream water is encountered." From this, it will be seen 

 that Verrill inclined to the opinion that the pebbles found in the 

 deep water were brought thither by floating ice, and if the ice 

 takes along pebbles, there is nothing to hinder for its also takiiiL! 

 along shells from the coast. 



in the Norwegian fiords, it is highly probable that drift-ice 

 causes a transportation of littoral shells out into the deep basins, j 

 For instance, Littorina rudis ami <>litii*siitn found at a depth of 

 150—180 m. on Risvserflaket, and Gibbula cineraria at a depth of 

 600 meters in the Oxsund, (in each case the shells were empty) 

 were neither of them in their primary locality. Hans Kxzer, 1 ) too, 

 has given a very plausible explanation of the storing of gravel and 

 shells in the snow and ice on the shore, and their conveyance to 

 places farther off when the ice melts in the spring: his explanation 

 is based upon personal observations at Tromso. 



Fauna and Hydrography. 



In the foregoing pages. I have tried to prove that there is a 

 close connection between fauna and hydrography. It would from 

 this again appear, that a majority of species of animals have an 

 organisation which can only bear a very slight variation in hydro- 

 graphical conditions. The number of socalled cosmopolitan species 

 is very small, and it seems to me that the number of those which 

 are mentioned as being widely distributed is also on the decrease. 

 The more exact morphological investigation which is demanded now- 

 a-days often results in the dividing of a species into two or more. 



And this is the case, not only with bottom forms, but also 

 with reference to plankton. 



I will give some instances of what I mean. We have for years 

 heard that Calanus finmarchicus is found in nearly every sea. But 

 G. 0. Saks-') now tells us that under the name C. finmarchicus 

 was hidden another species. C. helgolandicus, Clatjs, which is spe- 

 cially distributed southwards, while the former has an arctic and 

 boreal distribution. Strictly speaking, Calanus finmarchicus contained 

 three different species, for Calanus hyyerboreus was considered 

 to be a variety of C. finmarchicus previous to the publication of 

 Dr. Giesbrecht's well-known monograph on Copepods. Instead, 

 therefore, of one species distributed over nearly every sea. we now 

 gel three species with comparatively limited distribution, C. hyper- 

 boreus being arctic. C. finmarchicus boreal and arctic and C. hel- 

 golandicus lusitanic. 



Similarly with respect to Euchasta. Instead of the widely di- 

 stributed Euchmta norvegica, we now have, norvegica, glaeialis and 

 barbat'i. Examples might easily be multiplied from the Copepods. 

 On the other hand, it is beyond doubt that there are also deep- 



*) Niveauforandring eller transport. Naturen, 1902, p. 364. 

 -) Crustacea of Norway, Vol. IV. 



water forms of Copepods which have an exceedingly wide distri- 

 bution. Natural conditions are only subject to very slight changes 

 at the great depths, and this too for extensive stretches. It is also 

 very probable that there are shallow-water forms which are so 

 organized as to be able easily to adapt themselves to changes in 

 natural conditions, and are thus able to exist under very diverse 

 physical conditions, but their number has undoubtedly been over 

 rated. In his excellent monograph on northern Annulata, G. M. R. 

 Levinsen 1 ) says: — 



„1 must saj that I doubt whether the northern seas reallj 

 have so main species in common with the Mediterranean as would 

 appear from the lists uiven in the literature available." On account 



of this doubt. Levinsen carefully compared the 'thern forms and 



those from the Mediterranean, and come to the conclusion that Areni- 

 cola marina and Pectinaria Jcoreni from the latter sea were differ- 

 ent from the northern species bearing the same name. They were 

 given the names A. claparedi and /' robusta. 



Similarly with respect to Pectinaria auricoma, Potamilla reni- 

 formis etc. 



I could give a number of examples from the Bryozoa too, to 

 show how the extent of the distribution of a species diminishes, as 

 the claim for greater exactness in the determination of a species 

 increases. The change which has taken place in the use of the 

 word ..species" with regard to the Bryozoa, has had a similar effect. 

 F. A. SiriTT, for instance in his work on boreal ami arctic Bryo- 

 zoa. which in other respects is excellent, has used the word ..spe- 

 cies" in a very extended sense. He has entered as ..forms" a large 

 number of specimens which are given the rank of ..species" by 

 recent systematical investigators. 



As a result. Smitt's species were attributed with a much too 

 extensive geographical distribution. 



The distribution of a species is undoubtedly dependent on many 

 other things than the temperature and salinity of the water in which 

 it exists. Currents especially have both a direct and indirect in- 

 fluence. If one considers the conditions on the Norwegian coast, 

 where arctic and boreal fauna meet, the question naturally suggests 

 itself: - - Is it the arctic or the boreal animals which en our coasts 

 are gaining ground? 



To settle whether the movements of a uiven element of fauna 

 or flora are progressive or retrograde, one can examine the currents 

 in the adjoining sea. On the Norwegian coast, the current which flows 

 in a northerly direction is predominant, and the southern animal forms 

 are carried along with it. On the east coast of North America, 

 the reverse is the case. The duration of the pelagic state is also 

 important with regard to the penetration of the species into new 

 districts. It would therefore seem likely that such species as Myti- 

 lus edulis and Modiola modiolus, in which the pelagic state hardly 

 lasts much more than a week, would find it difficult to get over 

 the space between two coasts which arc separated by a wide 

 expanse of ocean. Along a length of coast line, however, these 

 and similar species are widely distributed, for. in the course of 

 thousands ,,f years, the many small steps forward amount to a 

 considerable distance. 



I do not know very much about the plankton in the more 

 southerly seas, but I have the impression that there is not so much 

 difference there in its quantity and quality at the different times 



') Systematisk geografisk Ovei igl over de nordiske Annulata, Gep} 

 Chcefognathi og Balonoglosni. Aftryk af Videnskab. Medd. fra den naturhist. 

 Poren. i Kjabenhavn 1882 og 1883 (p. 287). 



