HETEROGONY AND EVOLUTION 



223 



An exhaustive botanical study of the relation of size to 

 internal morphology has recently been published by Wardlaw 

 (1924-8), but the details are of a technical nature and I cannot 

 enter into them here. The conclusion which chiefly interests 

 us is that certain characteristic types of internal morphology 

 in vascular plants (e.g. polystely) need have no phyletic value, 

 since they appear to be modifications directly dependent upon 

 increase in size. These and other -'results have been sum- 



Fig. 96. — Changes in proportions during growth in wild sheep and improved 

 breeds of domestic sheep (see text). 



marized in a book by Bower (1930). The general conclusion 

 to be drawn from these botanical studies would seem to be 

 that in this case apparent orthogenesis is to be explained by 

 parallel adaptive evolution due to size-increase. 



We may take one example of the individual changes corre- 

 lated with size increase (Bower, I.e., p. 16). As we pass up 

 from rhizome to stem in Psilotum triquetrum, the size of the 

 organ increases. The size of the tracheidal tract also increases, 

 and its form also changes, exposing more surface than it would 



