PREFACE. xiii 



of the body ; such segments being composed of parts similar in 

 number and arrangement. Accordingly, a given part or appen- 

 dage in one segment is repeated in another, just as one bone is 

 represented in the skeletons of different Vertebrates; and this 

 representative relation in the segments of the same skeleton is 

 ' serial homology.' As, however, the parts can be namesakes 

 only in a general sense, as * centrums,' ' ribs,' &c, and since they 

 must be distinguished by special names according to their special 

 modifications, as ' basioccipital,' ' mandible,' * coracoid,' ' humerus,' 

 &c, I have called such serially related or repeated parts ' homo- 

 types.' j The basioccipital is the homotype of the basisphenoid, 

 and the humerus is the homotype of the femur : when the basi- 

 occipital is shown to repeat in its * vertebra ' the element which 

 the ' odontoid process ' represents in the succeeding vertebra, or 

 the basisphenoid in the preceding one, its ' serial homology ' is 

 indicated. 



The extent to which serial homologies can be determined 

 shows the degree in which vegetative repetition prevails in the 

 organisation of an animal. 



The study of homologies is comparatively recent ; much of this 

 field of research remains for future cultivators, especially in 

 regard to the muscular and nervous systems. 



When engaged in the l third way ' of anatomy, and in making 

 known the results of such labour as applied to the skeleton, 1 I 

 found a great impediment in the want of names of bones. For 

 these, when first studied, had been mostly described under phrases 

 suggested by forms, proportions, or likeness to some familiar 

 object, which they present in the human body. A reform of this 

 nomenclature was an essential first step, and it is gradually 

 making its way against the usual impediments. 



The best workman uses the best tools. Terms are the tools of 

 the teacher ; and only an inferior hand persists in toiling with a 

 clumsy instrument when a better one lies within his reach. But 

 i he has been used to the other.' No doubt ; and some extra 



1 On the Archetype and Homologies of the Vertebrate Skeleton, 8vo. 1848; and 

 On the Nature of Limbs, 8vo. 1849. 



