30 BIOLOGICAL LECTURES. 



forms as presented by Dr. Jeffrey in his recent admirable 

 memoir on " The Development, Structure, and Affinities of the 

 Genus Equisetum." 



Among the most interesting and promising of all botanical 

 problems is the theory relative to the origin of the sporophyte 

 as propounded by Bower a few years since, and more recently 

 emphasized by Campbell, who has himself done so much to 

 advance our knowledge in this direction. While the evidence 

 to be derived from living plants points with great force to the 

 possible correctness of Bower's hypothesis, it is not yet so 

 complete as to justify us in regarding the law as fully estab- 

 lished. But it is in the solution of problems of precisely this 

 nature that palaeobotany would prove of the highest importance, 

 and it is probably not too much to expect that the study of fos- 

 sil plants, particularly of types now extinct, may eventually 

 enlarge our views upon this as upon other important problems. 



If we now turn to plants as we find them in the rocks, it is 

 to be observed that the succession displayed by living forms is 

 there essentially repeated and thereby confirmed. A glance at 

 the geological succession of the earth's crust shows that there 

 are four great periods in which geological time may be reck- 

 oned, and that these periods conform in the main to epochs in 

 the development of plant life. In the earliest or Eozoic time, 

 chiefly represented in northeastern America by the great 

 Laurentian formation which gives the dominant physical as- 

 pect to the northern watershed of the St. Lawrence River 

 throughout the greater portion of its length, there are few and 

 trustworthy evidences of former plant life to be obtained ; 

 that is to say, we find in those rocks no well-defined plant 

 remains. On the other hand, the occurrence in the Palaeozoic 

 of plants of a somewhat high degree of organization leads to 

 the inference that they represent a line of descent which must 

 have had its origin very early in Eozoic time. But if definite 

 remains are wanting, there is nevertheless evidence in the 

 abundance of graphite which occurs in the Laurentian forma- 

 tion, commonly interstratified with gneiss and attaining a ver- 

 tical depth upwards of six hundred feet or more, of former 

 vegetation ; for, as Prestwich very correctly observes, there is 



