372 



PALEOZOIC SYSTEM OF ROCKS. 



woody cylinder of exogenous structure and gymnospermous tissue. And 

 if, as Williamson supposes,* many of the striated jointed stems called 



Calamites are only casts of the pith, the 

 stems must have been even much larger 

 than stated above. 



Thus, as Lepidodendrids connected 

 Lycopods with Conifers, and Sigillarids 

 connected Lycopods with Cycads, so 

 these connected Equisetaa with Conifers. 

 General Conclusion. — The conclusion 

 which we draw from this examination 

 of Coal plants is : 1. That they belong 

 to the highest Cryptogams, viz., Vascu- 

 lar Cryptogams, and the lowest Phamo- 

 gams, viz., Gymnosperms ; 2. That they 

 were intermediate between these now 

 widely-separated classes, and connected 

 them closely together. These facts are 

 strictly in accordance with the law 

 already announced (page 344), viz., that 



Fig. 513.— Fnnt Fig. 514.— Restoration J \r to /> > 



of caiamite of a caiamite (after the earliest representatives of any class 



(after Heer). 



Dawson). 



or order are not typical representatives 

 of that class or order, but connecting or comprehensive types — that is, 

 types which, along with their distinctive classic or ordinal characters, 

 united others which connected them with other classes or orders. 

 Thus the now widely-separated classes and orders of organisms, when 

 traced backward, in time approach each other more and more, and 

 probably unite in one common stem, although we are seldom able to 

 find the point of actual union. Thus, in this case, the now widely- 

 separated Cryptogams and Phaenogams, when traced backward, ap- 

 proach until in the Coal they are nearly, if not completely, united. 

 The organic kingdom may be compared to a tree whose trunk is proba- 

 bly to be found, if found at all, in the lowest strata ; its main branches 

 begin to separate in the Palaeozoic, the secondary branches in the Mes- 

 ozoic, and so the branching continues until the extreme ramification, 

 but also the flower and fruit, are found in the fauna and flora of the 

 present day. The duty of the evolutionist is to trace each bough 

 to its fellow-bough, and each branch to its fellow-branch, and thus 

 gradually to reconstruct this tree of life, and determine the law and the 

 cause of its growth. 



Nature, vol. viii, p. 447. 



