The Recapitulation Theory. 87 



Other writers have carried the principle of recapitulation much 

 farther, and notably Jeffrey (1917), who applies it to the histological 

 structures of vascular tissues, and further finds that regeneration 

 after wounding frequently leads to the reappearance of ancestral 

 characters. This principle is freely used by him in the comparison 

 of living with fossil forms, and in the interpretation of phytogenies, 

 To cite examples from the Araucarian conifers, it has long been 

 known (Thistleton-Dyer 1901) that in the adult stem of these trees, 

 owing to the continued activity of a cambium the leaf traces have 

 the peculiarity of being persistent for many years after the leaves 

 have fallen. Another well known peculiarity of Araucarian wood 

 is that the bordered pits in the xylem tracheids are alternate in 

 origin, not opposite as in other living conifers. Jeffrey (I.e., p. 236) 

 points out that in Mesozoic araucarian woods belonging to the 

 genus Brachyoxylon the leaf traces persist only for a short time, 

 and the bordered pits are not alternating and crowded as in the 

 living genera. In seedlings of the modern Agathis and Araucaria, 

 however, " the leaf trace persists only so long as it is related to a 

 functional leaf," and the pitting is like that found in the Cretaceous 

 Brachyoxylon. Such a striking case of recapitulation and others 

 of like nature are known can hardly find its explanation in a 

 germinal change which belongs equally to every cell. 



While these principles of recapitulation seem for the most part 

 well established, caution must of course be used in their application, 

 especially in these more complicated cases ; for it would be easy to 

 deduce incorrect phylogenetic conclusions by attaching more 

 significance to such cases than they really possess. It is important 

 also not to lose sight of the fact that recapitulation phenomena 

 occur where there has been adaptation to new conditions. Such 

 changes are often climatic, but may also be environmental in the 

 widest sense. 



One instance of more doubtful recapitulatory phenomena in 

 plants must suffice. In a recent paper on Rhododendron seedlings, 

 Professor Balfour (1917) has shown that many of the species in 

 their earlier years pass through a series of changes in the 

 pigmentation and pubescence of their leaves. Thus in R. adenogynwn 

 Diets the under surface of the leaves is red glandular in the seedling. 

 But about the third year the redness disappears as well as the 

 glandular hairs. After some seven years the buff-coloured 

 tomentum of long, interwoven, branched hairs begins to show at 

 the base of the (now green) leaves, and gradually in later years 



