DISTRIBUTION OP MARINE ALGiE. 167 



(and at a mean temperature of — 1°C), the Algse of that 

 region both grew normally and fructified ! It is well that 

 this observation should have weight, it is not crushing, — 

 that it may teach us caution in dealing with sweeping 

 rules. Let it be noted, however, that Kjellman's observa- 

 tion is no more potent than the exceptions I began by 

 citing, and that no rigid rule of zonal distribution is laid 

 down, and I think you will allow that a truth of general 

 value emerges from this cloud of witnesses against it. 



I have dwelt at some length on the details of zonal 

 distribution because they furnish us with a consideration 

 of some value. The natural classification of Algse is based 

 on the structure of the plants, especially valuable charac- 

 ters for this purpose being furnished by the organs of 

 reproduction, as in the case of other plants. However, 

 an artificial system founded solely on the colour of the 

 plants leads to a strikingly similar result. With certain 

 exceptions all the red Algse form a natural group, the 

 Khodophyceae, united and distinguished from other groups 

 by structural characters ; all the olive and brown form 

 another group, the Phaeophyceae ; all the green another, 

 the Chlorophyceae ; and all the blue-green another, the 

 Cyanophyceae, though the character of colour is of much 

 less importance here than in other groups. 



Allowing for exceptions under these heads, and they are 

 not numerous, under the first three at all events, we have 

 here a striking arrangement without a parallel in organ- 

 ized nature. Colour, so far from being a character of 

 value for the purpose of classification, is commonly of very 

 subordinate importance, and more often than not is even 

 deceptive as a mere specific character. To take a prom- 

 inent example, the genus Agaricus (mushrooms and toad- 

 stools) has been arranged in sections on a system largely 

 dependent for characterization on the colour of the spores ; 



