THE ALG^, OE TANGLES. 8 1 



amounts to about 70,000 feet. Now, as the thickness of all 

 the more recent superincumbent strata, from the Devonian 

 to the deposits of the present time, taken together, amounts 

 to only about 60,000 feet, we were enabled from this fact 

 alone to draw the conclusion — which is probable also for 

 other reasons — that the archilithic, or primordial, period was 

 of longer duration than the whole succeeding period down 

 to the present time. During the whole of this immeasur- 

 able space of time, which probably comprises many millions 

 of centuries, vegetable life on our earth seems to have been 

 represented exclusively by the svib-kingdom of Thallus 

 plants, and, moreover, only by the class of marine Thallus 

 plants, that is to say, the Algae. At least all the petrified 

 remains which are positively known to be of the primordial 

 period belong exclusively to this class. As all the animal 

 remains of this immense period also belong exclusively to 

 animals that lived in water, we come to the conclusion that 

 at that time organisms adapted to a life on land did not 

 exist at all. 



For these reasons the first and most imperfect of the great 

 provinces or branches of the vegetable kingdom, the division 

 of the Algse, or Tangles, must be of special interest to us. 

 But, in addition, there is the interest which this group 

 offers when viewed by itself In spite of the exceedingly 

 simple composition of their constituent cells, Avhich are but 

 little differentiated, the Algte show an extraordinary variety 

 of different forms. To them belong the simplest and most 

 imperfect of all fomis, as well as very highly developed and 

 peculiar forms. The different groups of Algae are dis- 

 tinguished as much by size of body as by the perfection and 

 variety of their outer form. At the lowest stage we find 



VOL. II. G . 



