CHAPTER XVII 

 THE CANONS OF COMPARATIVE ANATOMY 



With the completion of the consideration of the various tissues 

 and organs we are in a position to take up the relation of anatom- 

 ical structure to evolutionary sequence in the various groups of 

 vascular plants. Before we proceed to this phase of the subject, 

 however, it will be necessary to consider the general principles or 

 canons of comparative anatomy. It may be pointed out in this 

 connection that anatomy, in common with other branches of the 

 sciences, is based on inductive reasoning. The general principles 

 are consequently arrived at as the result of the consideration of 

 large numbers of facts with particular regard to the conclusions 

 which may be drawn from them. The anatomy of plants has 

 made great progress in recent years and in direct proportion to 

 our increasing knowledge of fossil forms. The most interesting 

 and valuable results from the evolutionary standpoint have been 

 reached in connection with the anatomical investigation of extinct 

 organisms of earlier geological ages. Naturally those of the great 

 coal-producing period, the Paleozoic, first received attention on 

 account of the importance of the study of plants of that age in 

 connection with the search for productive coal seams. In more 

 recent years the Mesozoic, which is of the greatest interest in 

 relation to the appearance of our modern types, has begun to be 

 studied. The results bearing on the advancement of our knowledge 

 of the general principles of the evolution of plants exemplified in 

 their anatomical structure cannot be too highly estimated. It will 

 be the aim of the present chapter to set forth comprehensively the 

 main conclusions of anatomical paleobotany in their relation to the 

 interpretation of the affinities of the main groups of vascular plants 

 now in existence on the earth. 



THE DOCTRINE OF RECAPITULATION 



An important general doctrine developed in connection with 

 the evolutionary study of living beings is the hypothesis of recapitu- 



