Il] FOSSIL PLANTS AND PHYLOGENY. 19 



This plant attained a height of fifty or a hundred feet, with a 

 proportionate girth, and increased in thickness in a manner 

 precisely similar to that in which our forest trees grow in 

 diameter. The exceptionally favourable conditions under which 

 specimens of calamitean plants have been preserved, have 

 enabled us to become almost as familiar with the minute 

 structure of their stems and roots, as well as with their 

 spore-producing organs, as with those of a living species. In 

 short, it is thoroughly established that Calamites agrees in most 

 essential respects with our well known Equisetum, and must be 

 included in the same order, or at least sub-class, as the recent 

 genus of Equisetaceae. As we ascend the geologic series from 

 the Coal-Measures, a marked numerical decline of Calamites 

 is obvious in the Permian period, and in the red sandstones 

 of the Vosges, which belong to the same series of rocks as the 

 Triassic strata of the Cheshire plain, the true Calamites is 

 replaced by a large Equisetum apparently identical in external 

 appearance and habit of growth with the species living to-day. 

 In the more recent strata the Horsetails are still represented, 

 but the size of the Tertiary species agrees more closely with 

 the comparatively small forms which have such a wide geo- 

 graphical distribution at the present time. Thus we are able 

 to trace out the history of a recent genus of Vascular Crypto- 

 gams, and to follow a particular type of organisation from the 

 time of its maximum development, through its gradual transition 

 to those structural characters which are represented in the 

 living descendants of the arborescent Calamites of the coal- 

 period forests. The pages of such a history are frequently 

 imperfect and , occasionally missiag, but others, again, are 

 written in characters as clear as those which we decipher 

 by a microscopical examination of the tissues of a recent 

 plant. 



As one of the most striking instances in which the micro- 

 scopic study of fossil plants has shown the way to a 

 satisfactory solution of the problems of development, we may 

 mention such extinct genera as Lyginodendron, Myeloxylon 

 and others. Each of these genera will be dealt with at some 

 length in the systematic part of the book, and we shall 



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