MACKIE — FIRST TEACES OF THE SUCCESSION OF LIFE. 



383 



as have two seed-lobes or cotyledons, as all forest-trees and slirubs, 

 whicli also are characterized by tbe possession of trne woody struc- 

 ture, the mode of growth being by concentric external layers aronnd 

 the stem, hence the term by which they are denominated — Exogens. 



The vegetable kingdom is, however, very variously grouped on 

 different and excellent principles by various authors ; but the gToup- 

 ing presented in the table below (Table III.) ^t11 be found suf&cient 

 for, and, we think, best adapted to, geological purposes. 



In our considerations of the lowermost of all fossiliferous rocks we 

 have brought under notice the first, or, at any rate, the oldest and 

 most remote forms of created beings as yet brought to light by the 

 researches of geologists. These, if we compare them with the posi- 

 tions of the classes to which they belong in the tables of the animal 

 and vegetable kingdoms, will be found to be not of the lowest gTades. 

 The Oldhamia, whatever it may really be, is certainly above the 

 monad or the diatom, and some naturalists have put it even as high 

 as the Seii;ularidse ; the worm-holes indicate the existence of a more 

 elevated class, the Annelida ; while frag-ments of trilobites carry us 

 still higher in the articulate gToup. Neither are the obscure vege- 

 table traces with which we are first presented at the bottom of their 

 kingdom, but they rank at least as high as the cellular algce. 



No foraminifers nor sponges, no animals of the globular t}^e, pre- 

 sent their remains ; no traces of diatoms, lichens, or fungi appear. It 

 may be said these were too perishable in their nature to be pre- 

 served. True some might have been so ; but others were not, for in 

 rocks less remote in age, diatoms, sponges, and foraminifers abound. 



When in Shropshire we pass away fi'om the Longmynds and reach 

 the well-known Stiper-stones ; or when in Wales, as at Harlech, we 

 pass from the Cambrian grits to the " Lingula-flags," fossils become 

 more abundant and more diversified. We have then entered into 

 another phase of the great Palaaozoic age, and new life-forms appear. 

 One of these, a brachiopod {Lingula Davisii) takes rank still higher 

 in the scale of hfe than any of the few forms met \^dth in the "Bottom- 

 rocks" and presents us with the first appearance of the molluscan 

 type ; while it occui^s in such abundance, and within such a zone-like 

 special range as to give a characteristic name to the rock-mass in 

 which it is embedded. 



