156 Biology in America 



wide, but temperature is usually, though not always, a very 

 effective barrier to their spread. This was strikingly illus- 

 trated by the disappearance of the tiiefish off the Atlantic 

 coast some years ago, which has been described in a previ- 

 ous chapter. Currents may serve not alone as a means of 

 transport for inactive forms, but through temperature dif- 

 ferences as a barrier to their spread, as well. Northern ani- 

 mals drifting southward in the Atlantic under the influence 

 of the Labrador current, sweeping past the shores of Labra- 

 dor and Newfoundland, may be caught by the Gulf Stream 

 moving toward the northwestern coasts of Europe, and their 

 southern journey terminated. 



During the cruise of the U. S. Bureau of Fisheries steamer 

 in 1884, Captain Tanner reports that on July 20, when off 

 the mouth of Chesapeake Bay "we passed numerous dead 

 octopods floating on the surface. This unusual sight at- 

 tracted immediate notice and no little surprise among those 

 who knew their habits, as it was not suspected at first that 

 they were dead. . . . These dead cephalopoda were seen fre- 

 quently on the 100-fathom line and outside of it, from the 

 position given above to the meridian of Montauk Point, a 

 distance of 180 miles. They were less numerous however as 

 we went to the northward and eastward. Several dead squid 

 were seen also, and two specimens were picked up with a 

 scoop-net." 6 



"From the Barents Sea we know many instances of a sim- 

 ilar destruction of animals on a large scale. The case of the 

 boreo-arctic fish, the capelan ... is specially striking, mil- 

 lions of this fish having occasionally been found drifting 

 dead at the surface. In the Barents Sea very sudden changes 

 of temperature occur, and it is natural to conclude that the 

 death of the fish is caused thereby. The greatest destruction 

 of this kind probably occurs among the young stages, eggs 

 and larvae of fishes. As we shall see later, these young 

 stages may be removed by currents very far from the places 

 where they are capable of developing, and in all probability 

 they are liable to encounter catastrophes which sweep them 

 off in enormous numbers. ' ' 7 



So too the chemical environment prevents the invasion of 

 inland waters by marine forms and vice versa, although this 

 is not true of those fish like the shad and salmon, which 

 ascend the rivers at spawning time. In such cases however 

 the age and sexual maturity of the fish determine their move- 

 ments, so that at most times of the year with these fish as well 



Tanner, "Keport of U. S. Fish Commissioner for 1884," p. 32. 

 'Murray, "The Depths of the Ocean," pp. 707-8. By permission of 

 the Macmillan Company. 



