ANIMAL AFFINITIES 283 



brates and vertebrates) ; respiration by means of lungs or gills 

 (mammals, sauropsida, ichthyopsida) ; bivalved or spiral shells 

 (lamellibranch and gasteropod molluscs) and so on. These 

 tectonic characters are, as a rule, unique, the occurrence of each 

 diagnosing some one category of animals. They are the mani- 

 festations of body-building. They are usually not apparent on 

 mere inspection of the intact animal but must be discovered by 

 dissection and by investigation of the developmental history. 



93. ON HOMOLOGIES 



Such structures as the notochord of an Amphioxus and that 

 of a Hag-fish are said to be homologous. The swim-bladder of 

 a fish and the lungs of a mammal ; the endostyle of a Tunicate 

 and the thyroid gland of a mammal ; the flipper of a whale and 

 the forelimb of a rabbit ; the pineal gland in man and the median, 

 undeveloped eye of a lizard — these are all pairs of homologous 

 structures. The blastodermic vesicle, in the development of a 

 mammal and the blastula of an amphioxus are homologous phases 

 in embryogeny. 



93^. The Criterion of Homology. Structures that have 

 the same origins and initial modes of development are said to be 

 homologous. In a general way an established homology suggests 

 that the structures involved have a fundamental significance that 

 is the same, but we can only give precision to this notion by 

 making the above definition. We stipulate, then, that the 

 criteria of the homology of two structures are their similarities 

 of development. 



936. Tectonic Chail\cters express Homologies. The char- 

 acters that are used in the major classificatory systems are tectonic 

 ones and involve homologies. Thus the notochord, an axial 

 stiffening rod, is of great tectonic importance in the structure 

 of the bodies of very many animals. Wherever we find this 

 structure and are able to trace its development we find that this 

 is always the same — the notochord originates in embryogeny 

 as an invagination of endoderm. Therefore it is homologous 

 in all chordate animals. On the other hand, the eyes of the 

 mollusc, Pecten, and those of the vertebrates exhibit certain 

 curious resemblances, but these organs develop in very different 

 ways and so we say that they are not homologous structures. 



