THE CANONS OF COMPARATIVE ANATOMY 243 



which ordinarily manifest themselves, not in the actual wound cap 

 or hypertrophied mass of wood formed after injury, but on the 

 opposite side of the stem. Among the dicotyledons the birch 

 exemplifies the same condition in regard to the formation of aggre- 

 gate rays. The diffuse structure of radial parenchyma which 

 characterizes the normal organization of the wood of the genus 

 Betula gives way to a condition of aggregation following injury, not 

 on the edges of the wound (that is, in the actual wound cap), but 

 in the region of the axis diametrically opposite to the injury. In 

 accordance with the general reversionary principles here stated, 

 the wound cap as often as. not presents structures in an advanced 

 or accelerated, and not in a reversionary, condition. Some recent ! 

 publications on the anatomy of the dicotyledons reveal a failure to 

 realize this fundamental principle, and therefore it is well that it 

 should be emphasized in the present connection. 



The foregoing paragraphs elucidate the most important canons 

 or principles of evolutionary anatomy. It cannot be too strongly 

 urged that all the evidence avaikble under the various principles 

 here described and exemplified should.be brought into considera- 

 tion, as a failure to check up one kind of evidence against another 

 often results in a fallacious and ephemeral deduction. The most 

 cogent testimony to the validity of any evolutionary conclusion is 

 naturally supplied by the conditions actually realized in ancestral 

 fossil forms. Since, however, by reason of the incompleteness of 

 the geological record and our consequent ignorance of the organiza- 

 tion of older forms we are frequently not in the position to avail 

 ourselves of the actual past history of given plant types, we must of 

 necessity have recourse to the valuable aid furnished by the 

 important general principles described in the earlier paragraphs 

 of the present chapter. Where the canons of evolutionary anatomy 

 are judiciously employed, the result is usually so clear and convin- 

 cing as to commend itself to the unprejudiced mind. 



