THE CANONS OF COMPARATIVE ANATOMY 237 



gymnosperms have come from ancestors possessing concentric 

 bundles and centripetal wood, yet the seedlings of cycads in general 

 do not support this conclusion by their anatomical organization. 

 The doctrine of recapitulation is of value, accordingly, when it 

 presents positive evidence from the seedling for the ancestral 

 occurrence of a given feature of organization; but negative testi- 

 mony from this standpoint must be estimated as having little or 

 no value. A failure to realize this situation is responsible for much 

 fallacious biological reasoning. 



A very important exemplification of recapitulation is frequently 

 supplied by the first annual ring of the older stem of arboreal forms. 

 Often in groups which have suffered considerable reduction, such 

 as, for example, the gymnosperms in general, the phenomenon of 

 recapitulation, although absent in the seedling, may be clearly 

 illustrated by the first annual increment of woody growth in the 

 older regions of the stem. An illustration of this principle is 

 supplied by the living araucarian conifers. Taking as an example 

 the genus Agathis, the kauri of Australasia and the East Indian 

 region, we find in the first annual ring an organization distinctly 

 different from that in the subsequent annual increments of the 

 wood. More or less abundant wood parenchyma is present, 

 although longitudinal storage elements are conspicuous by their 

 absence in the adult wood of the stem. This situation is of great 

 interest in view of the fact that the fossil wood of the kauri from 

 American Cretaceous deposits is characterized by the presence of 

 parenchymatous cells, not only in the first annual ring, but in all 

 subsequent zones of ligneous growth. The persistence of the 

 structure of Mesozoic forms in the first annual ring of living species 

 of the genus Agathis is a feature most appropriately falling under 

 the principle of recapitulation. The situation here indicated is of 

 great value and wide validity, not only for the gymnosperms, but 

 also for the dicotyledons. It might readily be much more abun- 

 dantly exemplified in the present connection, but the instance 

 supplied above will serve to make the situation clear. Many other 

 cases will present themselves in later chapters in connection with 

 the discussion of the evolution of the different groups as inferred 

 from their anatomical organization. 



