OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
231 
the fossil forms, we find that the stems of the Catamites, the approximate represen- 
tatives of the Equisetaceas, and those of the Lepidodendra with their extreme Sigillarian 
modifications, are of the exogenous type, whilst those of ferns are, in the points referred 
to, as obviously endogenous. The respective affinities of these plants, so far as the stems 
are concerned, may be represented by some such diagram as the following. 
It will be observed that in this memoir I have paid but little attention to generic 
distinctions and none to specific ones, because I am satisfied that we are not yet in a 
position to define either the one or the other. My object has been to ascertain, as far 
as I could, what are the principal types of structure, and what the ranges of their vari- 
ation ; but, on the latter point especially, very much remains to be done which can only 
be accomplished by the cooperation of multiplied observers, and especially of such as are 
investigating distinct localities where new varieties may be expected to obtain. By such 
observations alone can our mutual errors and oversights be corrected. Where examples 
of plants in which structure is preserved are rare, we are in danger of drawing general 
conclusions from individual varieties which happen to be sharply defined : hence it is 
most important that independent observers should not be deterred from again going 
over the ground by an idea that it is preoccupied or that the work is done. The pre- 
sent contribution, however, carefully executed as far as it goes, is but that of a pioneer 
in a very wide and almost unexplored field. 
It only remains for me to acknowledge the assistance which I have received either in 
the loan of sections or, what has been of even greater value to me, of specimens for dissec- 
tion. The gentlemen to whom I have been thus indebted are W. Boyd Dawkins, Esq., 
E.B.S., of Manchester, Mr. J. Butterworth, of Shaw, and Mr. Whittaker and Mr. Nield, 
of Oldham. The scientific liberality of my two last-named auxiliaries demands special 
notice. They have not only given me the freest access to their cabinets, but have allowed 
me to cut into fragments some of the choicest specimens which they contained, when 
the interests of scientific truth seemed to demand the sacrifice. Such a spirit is too rare 
not to merit the thanks of all investigators whenever it is met with. 
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