DISTRIBUTION OF MAEINE ALG^E. 169 



true of seaweeds than of the higher plants. The complex 

 conditions that affect the distribution of land plants, and 

 render it no mere affair of latitudes or even of isothermal 

 lines, are fewer, so far as we know, in the case of marine 

 plants, but some of them are far more potent. Local 

 peculiarities play an important part. The fact that certain 

 seas are tideless, for example, profoundly affects the 

 character of their vegetation. The Mediterranean, to 

 take an instance, does not possess a considerable number 

 of forms which might be reasonably expected to occur in 

 it were there an average rise and fall. So also with the 

 Antilles, rich as they are in marine plants. As agents in 

 distribution, let us take the case of the great ocean currents. 

 They are not merely vehicles for transport, but they are 

 streams of stable temperature as well. In all probability 

 — though this is a matter on which we have little positive 

 knowledge — seaweeds are more sensitive to fluctuations 

 of temperature than are land plants. (There are cosmo- 

 politan species of course, but these are out of consideration 

 at present.) The conditions that surround seaweeds 

 living in a medium of stable temperature such as the sea 

 — not called upon to endure much change, in that respect, 

 from day to night nor even from season to season — en- 

 courage this opinion. The difficulties that attend the 

 procuring of the germination and the culture of seaweeds in 

 aquaria are rightly suspected to spring from inaccurate 

 adjustment of temperature. More than this, direct obser- 

 vation of the distribution of seaweeds within limits of 

 temperature in the sea shows us that this is the case. 

 We may, therefore, regard the great ocean currents as 

 prime movers in the transport of marine vegetation. Let 

 us take the familar instance of the Gulf Stream. As it 

 leaves the shores of America we may note the contrast 

 between the Florida Keys and Charlestown, not far to 



