Divergent views have been held by different authors both as regards the nature of the cormus as a whole, and as regards the homologies of the different types of appendages borne by it.
From the rough comparison of the skeleton of a bird with that of a man by Pierre Delon, in the 16th century (to go no further back), down to the theory of the limbs and the theory of the skull at the present day; or, from the first demonstration of the homologies of the parts of a flower by C. F.
The introduction of the phylogenetic factor has very much increased the difficulty of determining homologies; for the data necessary for tracing phylogeny can only be obtained by the study of a series of allied, presumably ancestral, forms. One of the chief difficulties met with in this line of research, which is one of the more striking developments of modern morphology, is that of distinguishing between organs which are reduced, and those which are really primitive.
Rolleston, " On the Homologies of Certain Muscles connected with the Shoulder-joint," Trans.
The situation of the lateral nervestems in the different genera with respect to the muscular layers lends definite support to the interpretation of their homologies here given and forms the basis of Burger's classification.