XVII] THE COMPARISON OF RELATED FORMS 757 



that it may also be employed for drawing hypothetical structures, 

 on the assumption that they have varied from a known form in 

 some definite way. And this process may be especially useful, 

 and will be most obviously legitimate, when we apply it to the 

 particular case of representing intermediate stages between two 

 forms which are actually known to exist, in other words, of recon- 

 structing the transitional stages through which the course of 



Fig. 388. Pflvis of Archaeopteriix. 



Fig. 389. Pelvis of Apalornis. 



evolution must have successively travelled if it has brought about 

 the change from some ancestral type to its presumed descendant. 

 Some little time ago I sent to my friend, Mr Gerhard Heilmann 

 of Copenhagen, a few of my own rough co-ordinate diagrams, in- 

 cluding some in which the pelves of certain ancient and primitive 

 birds were compared one with another. Mr Heilmann, who is 

 both a skilled draughtsman and an able morphologist, returned 

 me a set of diagrams which are a vast improvement on my own, 



