1070 THE THEORY OF TRANSFORMATIONS [ch. 



are actually known to exist, in other words, of reconstructing the 

 transitional stages through which the course of evolution must have 

 successively travelled if it has brought about the change from some 

 ancestral type to its presumed descendant. Some years ago 

 I sent my friend, Mr Gerhard Heilmann of Copenhagen, a few of 



33*S6 7e<f 



Fig. 5,32. Pelvis of Archaeopteryx. 



my own rough coordinate diagrams, including 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 



Fig. 533. Pelvis of Apatornis. 



a vast improvement on my own, and which are reproduced in 

 Figs. 532-537. Here we have, as extreme cases, the pelvis of 

 Archaeopteryx, the most ancient of known birds, and that of Apa- 

 tornis, one of the fossil "toothed" birds from the North American 

 Cretaceous formations — a bird shewing some resemblance to the 

 modern terns. The pelvis of Arckaeopteryx is taken as our type, 

 and referred accordingly to Cartesian coordinates (Fig. 532) ; while 



