THE ALG, OR TANGLES, SI 
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 sub-kingdom of Thallus 
plants, and, moreover, only by the class of marine Thallus 
plants, that is to say, the Algz. 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 Algz, 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, which are but 
little differentiated, the Algze show an extraordinary variety 
of different forms. To them belong the simplest and most 
imperfect of all forms, as well as very highly developed and 
peculiar forms. The different groups of Alo 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 
