VIl] LARGE SEAWEEDS. 139 



the importance of the geological history of these plants by the 

 number of recorded fossil species, we should arrive at a totally 

 wrong and misleading estimate. By far the greater number 

 of the supposed fossil algae have no claim to be regarded as 

 authentic records of this class of Thallophytes. It has been 

 justly said that palaeontologists have been in the habit of 

 referring to algae such impressions or markings on rocks as 

 cannot well be included in any other group. " A fossil alga," 

 has often been the dernier ressort of the doubtful student. 



Before discussing our knowledge, or rather lack of know- 

 ledge, of fossil algae at greater length, it will be well to briefly 

 consider the manner of occurrence and botanical nature of ex- 

 isting forms. In the sea and in fresh water, as well as in damp 

 places and even in situations subject to periods of drought, 

 algae occur in abundance in all parts of the world. We find 

 them attaining full development and reproducing themselves at 

 a temperature of — 1° C. in the Arctic Seas, and again living in 

 enormous numbers in the waters of thermal springs. Around 

 the coast-line of land areas, and on the floor of shallow seas 

 algae exhibit a remarkable wealth of form and luxuriance of 

 growth. As regards habit and structure, there is every grada- 

 tion from algae in which the whole individual consists of a 

 thin-walled unseptate vesicle, to those in which the thallus 

 attains a length unsurpassed by any other plant, and of which 

 the anatomical features clearly express a well-marked physio- 

 logical division of labour such as occurs in the highest plants. 



The large and leathery seaweeds which flourish in the 

 extreme northern and southern seas are plants which it is 

 reasonable to suppose might well have left traces of their 

 existence in ancient sediments. Sir Joseph Hooker, in his 

 account of the Antarctic flora^ investigated during Sir James 

 Ross's voyage in H.M. ships Erebus and Terror, has given 

 an exceedingly interesting description of the gigantic brown 

 seaweeds of southern latitudes. The trunks are described as 

 usually 5 — 10 feet long, and as thick as a human thigh, dividing 

 towards the summit into numerous pendulous branches which 

 are again broken up into sprays with linear 'leaves.' Hooker 

 1 Hooker, J. D. (44) p. 457. Pis. OLXvn. olxviii. and olxxi. D. 



