THE CANONS OF COMPARATIVE ANATOMY 241 



present work. Here the lack of complexity of organization rather 

 militates against the presence of phylogenetically important 

 structures. It must nevertheless be noted that the sporangium is 

 an extremely conservative organ and, as far as the relative simplicity 

 of its organization supplies points of comparison, is of very great 

 significance for the doctrine of evolution. It has been demonstrated 

 in an earlier chapter that the sporangium perpetuates the protean 

 centripetal xylem in the form of its opening mechanism to a higher 

 point than any other organ but the root. The value of the spore 

 sac in phylogeny, although limited by the relative simplicity of 

 the organ, must consequently be estimated as great. 



The doctrine of conservative organs is of the greatest signifi- ^ 

 cance in connection with the study of the evolutionary history 

 of plants, because of the abundance and reliability of the evidence 

 which the various parts furnish in this connection. Obviously, if 

 leaf, reproductive axis, root, and sporangium all supply consonant 

 and harmonious testimony in the same direction, a sound con- 

 clusion must inevitably be reached. There can be little question 

 that the doctrine of conservative organs is the most important one 

 which modern inductive anatomy has supplied as a tool of evolu- 

 tionary investigation. In fact, the general principles included 

 under this head are of such great significance that the present 

 volume may be considered as written only for those whose anatom- 

 ical training has progressed to such a point that they are able to 

 appreciate the universal value and validity of the doctrine here 

 discussed. 



THE DOCTRINE OF REVERSION 



This doctrine is well shown in the case of plants with a consider- 

 able amount of secondary growth namely, the gymnosperms and 

 the dicotyledonous angiosperms. It is of little value in herba- 

 ceous forms, whether cryptogamic or phanerogamic, since in these 

 the effects of injury are usually in the direction of degeneracy 

 only. In plants with secondary growth and consequently more 

 massive organs the effect of injuries frequently is to recall ances- 

 tral features of organization. This phenomenon is called reversion. 

 Only in certain conifers can we observe the effect of injury in 

 recalling in living forms features which are known to have been 



