i 



1881-82.] EdinbitrgJi Naturalists' Field Cbib. 17 



mentioned. Besides, the researches of Bendant and Plateau have 

 proved that it is possible to accustom fresh-water species to live in 

 the sea, and marine species to live in fresh water. The experi- 

 ments of Schmawkewitch show that "by greatly increasing the 

 saltness of the water, the crustacean Artemia salina became trans- 

 formed in the course of a few generations into the totally dissimilar 

 species Artemia Mulhauseni ; while by the converse process he 

 succeeded in transmitting the latter back again into the former. 

 A still more extraordinary circumstance followed the dilution until 

 it was perfectly fresh of the salt water in which Artemia salina lives, 

 for in the course of a few generations the character of that species 

 became so changed that they finally assumed those of a different 

 genus, Branchipus." Can we draw an analogy ? Any two of these 

 crustaceans may have been derived from the other by the medium 

 in which they lived being altered by natural means, and not by 

 artificial methods as in the experiments. Similarly, one species 

 existing in a habitat iu a gradual state of conversion into another 

 may become eventually specifically different from the same plant, 

 wliich may still exist in some remote part in its original state. 



Upheaval of the sea-bottom in the direction of currents flowing 

 from continents will tend to extend the flora of that continent — at 

 least, of the parts opposite the spot where upheaval has taken 

 place. Thus the Bermudas, lying 800 miles east of the coast of 

 Virginia, and placed in the track of the Gulf Stream, have not a 

 single indigenous plant. They are all similar to those found on 

 the opposite coast of America. A contrary case is that of the 

 Mauritius, lying off the east coast of Africa, which has not a single 

 plant similar to those on the opposite African shore, owing to the 

 fact that no currents are said to run between the two spots. One 

 tropical plant, at least, is known to have been found in a germinating 

 condition on the southern shores of England ; and two American 

 plants, the Anacharis and Mimulus, have lately taken a strong hold 

 in this island. The time during which man has studied nature in 

 an intelligible manner has been so short, that there is no wonder 

 we have not learned whence many of our native plants have come, 

 and what links in the chain of life have been broken. The direc- 

 tion of sea-currents must be dependent in some measure on the 

 distribution of the land. The present distribution of land and 

 water has not always existed ; and to cite one case only, the de- 

 pression of the land-surface of Britain before the deposition of the 

 Chalk must have been very great. Equally as great is the height 

 to which the secondary rocks of the Alps have been upheaved. 

 If the theory of the Weald be correct, then we once had a river 

 running through Britain as large as the Ganges, and therefore 

 draining a large part of land now covered by the Atlantic Ocean. 

 As islands lying off tlie coast of a continent generally possess a 



VOL. I. B 



