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POPULAR SCIENCE MONTHLY 



to plant them here and there in good soil at 

 considerable distances from where any have 

 heretofore been grown. " For a time ene- 

 mies do not find them." Pear trees planted 

 scatteringly are more likely to remain healthy 

 than in orchards. " Perhaps one reason 

 why plants have become extinct or nearly so 

 is their lack of means of migration. As ani- 

 mals starve out in certain seasons when food 

 is scarce, or more likely migrate to regions 

 which can afford food, so plants desert worn- 

 out land and seek fresh fields. As animals 

 retreat to secluded and isolated spots to es- 

 cape their enemies, so many plants accom- 

 plish the same thing by finding the best 

 places with some of their seeds sown in many 

 regions. Frequent rotations seem to be the 

 rule for many plants when left to themselves 

 in a state of nature. Confining to a perma- 

 nent spot invites parasites and other enemies 

 and a depleted soil, while health and vigor 

 are secured by frequent migrations." 



Commensals. — Curious associations are 

 formed among animals for mutual aid in the 

 struggle for existence. Some of them are 

 societies of the same species, like those of 

 ants and bees ; colonies in which many indi- 

 viduals — as ascidians and bryozoa — join into 

 a single mass and act as one ; and associa- 

 tions of animals of different species consti- 

 tuting commensalism where both are bene- 

 fited, or parasitism, when the advantage 

 accrues to only one of the parties. The her- 

 mit crab and certain ascidians furnish very 

 fine examples of commensalism. The her- 

 mit crab is known as an inhabitant of shells 

 bereft of their proper owners. Some sea 

 anemones also fasten themselves on shells, 

 and seem to prefer those which have been 

 adopted by hermit crabs. The association is 

 shown by M. Henri Coupan, in La Nature, to 

 be one of mutual benefit. The actinia de- 

 fends the crab and its home against all in- 

 truders by means of its tentacles — veritable 

 batteries of prickly stings ; while the crab, 

 with its long claws reaching out to catch 

 whatever is good to eat, brings food within 

 reach of the ascidian. Mr. Percival Wright, 

 having taken the crab from a shell to which 

 an ascidian had attached itself, found that 

 the latter abandoned the shell in a short 

 time. M. L. Faunt reversed the experiment, 

 taking the ascidian away, when the crab de- I 



serted its quarters, found a shell with the as- 

 cidian on it, and occupied it very quickly. 

 He further observed the maneuvers executed 

 by the crab to secure the attachment of an 

 ascidian to its shell. Sometimes a large as- 

 cidian will wholly cover a shell ; or several 

 smaller ones will spread themselves over the 

 same shell so as to form a continuous en- 

 velope over it. The ascidians become so at- 

 tached to their commensals as to seem un- 

 able to live without them, and even to die 

 soon after being separated from them. 



Drift of Ocean Currents. — Of sixteen hun- 

 dred and seventy-five floats bearing requests 

 to the finder to return them which Prince 

 Albert of Monaco dropped into the Atlantic 

 during three research cruises, with a view to 

 learning something of the movements of sur- 

 face currents, two hundred and twenty-six 

 were returned to him up to the year 1892. 

 By working the course which each of them 

 had probably been following, the prince un- 

 dertook to draw a definite map of the cur- 

 rents. As the elements employed were al- 

 ways numerous for each region, he thinks 

 his results were near the truth in its general 

 lines. The floats landed on almost all the 

 shores of the North Atlantic, from the North 

 Cape to the south of Morocco, along Central 

 America, and on the islands of Canaries, 

 Madeira, Azores, Antilles, Bermudas, Shet- 

 land, Hebrides, Orkneys, and Iceland. None 

 appeared as far south as the Cape Veid 

 Islands. The drifts seem to indicate an im- 

 mense vortex, beginning toward the Antilles 

 and Central America with the Gulf Stream 

 and the equatorial current ; passing the Banks 

 of Newfoundland at a tangent, it turns to 

 the east, approaches the European coasts, 

 and runs southward from the English Chan- 

 nel to Gibraltar, after having sent a branch 

 running along the coast of Ireland and the 

 coast of Norway as far as the North Cape. 

 It then returns to the west, encircling the 

 Canaries. Its center oscillates somewhere to 

 the southwest of the Azores. The author's 

 observations enabled him also to establish a 

 very good average for the speed at which 

 these floats traveled in the different sections 

 of the vortex, and for every twenty-four 

 hours : Between the Azores, France, Portu- 

 gal, and the Canaries, it was 5.18 miles; 

 from the Canaries to the Antilles, the Baha- 



