HA RD WICKE'S SCIENCE- G OS SIP. 



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restricted, either, as in those which inhabit many 

 countries and both hemispheres. In plants, the genus 

 Potamogeton, for instance, is such as to puzzle the 

 most skilful, and even in Ranunculus, the terrestrial 

 forms of which are fairly well-defined, the aquatic 

 species or sub-species defy classification. 



Of freshwater fish, the Salmonidre offer sufficiently 

 good examples of variation ; for instance, the trout of 

 Great Britain and Ireland, all referred by Dr. Day to 

 Sal mo trutta, are divided by Dr. Giinther into no less 

 than eleven species ; so that it evidently becomes 

 a question, as with most other freshwater genera — Are 

 we to say that the genus is a large one including 

 many species of very restricted range, or a small one 

 with few species of very wide range? 



The Trichopterous Philopotamns montanus, again, is 

 an analogous case. This insect has a variety, Scoticus, 

 which Mr. King found near Killarney, in the same 

 district as the typical form, but confined to a single 

 stream, where the type did not occur. Further, 

 there are two other] forms of Philopotaimis, cesareus 

 and insularis, peculiar to Jersey and Guernsey 

 respectively, and which, it would appear, may either 

 be considered species or varieties of P. montanus, 

 according to the view taken of the limitations of 

 these terms. 



It seems therefore to me, reviewing all these facts, 

 that the bare statement that freshwater forms are very 

 widely distributed does not convey the exact truth of 

 the matter ; but rather we should say that the races 

 are not so well defined, and merge more into one 

 another than is the case with terrestrial forms, and 

 hence what might, if all connecting links were 

 destroyed, be considered a genus of many species, 

 becomes a single widely-distributed species with 

 endless varieties. 



The explanation of this may probably lie in the 

 fact, that freshwater species cannot spread as 

 gradually as if they were on land ; but are conveyed 

 from pond to pond, river to river, or river to pond, as 

 chance may have it, and nearly always the new 

 habitat differs somewhat from the old, whether in 

 the flow or constitution of the water, or the nature 

 of the food, so that it is clear that only the most 

 pliable species, which readily adapt themselves 

 to new conditions, would stand a good chance of 

 surviving. 



This theory may throw some light on the extra- 

 ordinary difference between the freshwater mussels 

 ( Unio and Anodonta) of Europe and America. In 

 Europe we have a few species belonging to each 

 genus, presenting innumerable and puzzling varieties, 

 but in America hosts of more or less well-defined 

 species. This enormous abundance of specific forms 

 in America as compared with Europe has been 

 attributed to the diversity of geological changes 

 supposed to have occurred in America, and the 

 consequent isolation of forms. I attribute it to 

 precisely reverse conditions — the great basin of the 



Mississippi, in which most of the species arc found,, 

 is connected throughout, and nearly fulfils the 

 conditions of a terrestrial area in the opportunities it 

 offers for gradual migration ; whereas in Europe there 

 are many river systems, distinct, yet of no great size,, 

 such as those of the Seine, the Thames, the Rhone, 

 the Garonne, and an endless host, all of which must 

 be peopled from each other by sudden and not by 

 gradual migration. 



Paradoxical as it may seem, that isolation should 

 prevent the development of many species, I am forced' 

 to this conclusion as to freshwater forms by the- 

 facts before me, and I shall look eagerly for any 

 comments which naturalists may be able to afford 

 on the subject, tending to prove or disprove my 

 conclusions. 



Now to come to the second part of my subject, the- 

 distribution of species inhabiting the sea. Those of 

 the British coasts may be divided roughly into 

 northern and southern, yet their origin must have 

 been from many sources, which we can only dimly 

 trace at present. A resemblance has been noted 

 between the marine Crustacea of South America and. 

 of Britain, which may possibly be due to the Gulf 

 Stream peopling our shores with types from America,, 

 while it seems not unlikely that land once existed at 

 an intermediate point, if there was no terrestrial 

 connection. 



Floating species are sometimes world-wide. I have 

 seen examples of Spirilla peronii and Ianthina exigita- 

 from New Zealand, and these same shells are occasion- 

 ally washed up on the western coasts of the British 

 Isles. The marine mollusca of the northern and 

 southern temperate zones bear a remarkable likeness- 

 to one another, being for the most part of identical 

 genera and very similar in appearance, though there 

 are of course many notable exceptions. 



The distribution of many species of univalve 

 mollusca appears to be very largely influenced by 

 warm and cold currents of water, as the Rev. A. H. 

 Cooke pointed out to me in the case of the genus 

 Purpura, in which this obtained both in the Atlantic 

 and the Pacific. Central France was at one time 

 probably submerged, and an open connection existed 

 between the Bay of Biscay and the English Channel 

 and the Mediterranean, as seems probable from the 

 Mediterranean character of many of the pliocene 

 fossils found at St. Erth, Cornwall, and from Dr.- 

 Gwyn Jeffreys' account of the deep sea mollusca of 

 the Bay of Biscay. — All these various points are 

 worthy of careful and detailed investigation ; I merely 

 mention them here, but their consideration in detail 

 would fill a book. 



Boreal marine species are circumpolar, and, fre- 

 quently following the coast-line of continents, appear 

 where one would hardly expect them. Let us take,, 

 for instance, the Pacific coast of North America. 

 W. H. Dall gives a list of the mollusca found on the 

 expedition to Point Barrow, Alaska, and this includes 



