174 REPORT — 1846. 



stood ' analogous development,' one cannot determine how much or how little 

 it is applicable to the determination of homologies or to the definition of 

 homologous parts. Dr. Reichert seems to have been unduly influenced by the 

 idea of ' analogy or similarity of development in the determination of homo- 

 logous parts ' when he rejected the parietal and frontal bones from the system 

 of the endo-skeleton, because they were not developed from a pre-existing 

 cartilaginous basis*, or, because they could be easily detached from subja- 

 cent persistent cartilage in certain fishes ; the essential distinction between 

 these and the supra- occipital in regard to development being, that whereas 

 the cartilaginous stage intervened in the latter between the membranous and 

 the osseous stages, in the other, usually more expanded, cranial spines, the 

 osseous change appears to be immediately superinduced upon the primitive 

 aponeurotic histological condition. 



M. Agassiz seems, in like manner, to give undue importance to similarity 

 of development in the determination of homologies, where he repudiates the 

 general homology of the basi-sphenoid with the vertebral centrum, and con- 

 sequently its serial homology with the basi-occipital, because the pointed end 

 of the chorda dorsalis has not been traced further forwards along the basis 

 of the cranium in the embryo osseous fish than the basi-occipital f. But the 

 development of the centrum of every vertebra begins, not in the gelatinous 

 chord, but in its aponeurotic capsule, and it is in the expanded aponeurosis 

 directly continued from the 'chorda' along the 'basis cranii ' that the thin 

 stratum of cartilage cells is formed from which the ossification of the basi- 

 sphenoid, presphenoid and vomer proceeds. 



There exists doubtless a close general resemblance in the mode of deve- 

 lopment of homologous parts ; but this is subject to modification, like the 

 forms, proportions, functions and very substance of such parts, without their 

 essential homological relationships being thereby obliterated. These rela- 

 tionships are mainly, if not -wholly, determined by the relative position and 

 connection of the parts, and may exist independently of form, proportion, 

 substance, function and similarity of development. But the connections 

 must be sought for at every period of development, and the changes of rela- 

 tive position, if any, during growth, must be compared with the connections 

 which the part presents in the classes where vegetative repetition is greatest 

 and adaptive modification least. 



Relations of homology are often not only confounded with those of analogy, 

 but in some recent and highly estimable works on comparative anatomy the 

 terms ' analogy ' and ' analogue ' continue to be used to express the ideas of 

 homology and homologue, or are so used as to leave in doubt the meaning of 

 the author. Thus when we read in the latest edition of the ' Lecons d'Ana- 

 tomie Comparee ' of Cuvier, " Les branchies sont les poumons des animaux 

 absolument aquatiques," t. vii. p. 164; and with regard to the cartilaginous 

 or osseous supports of the gills, " elles sont, a notre avis, aux branchies des 

 poissons, ce que les cerceaux cartilagineux ou osseux des voies aeriennes sont 

 aux poumons des trois classes superieures," Ibid. p. 177, we are left in doubt 

 whether it is meant that the gills and their mechanical supports merely perform 

 the same function in fishes which the lungs and windpipe do in mammals, or 

 whether they are not also actually the same parts differently modified in re- 

 lation to the different respiratory media in the two classes of animals. The 

 deeper-thinking Geoffroy leaves no doubt as to his meaning where he argues 



* Vergleichende Entwickelungsgeschichte des Kopfes der nackten Reptilien, 4to, 1838, 

 pp. 212, 218. 



f Recherches sur les Poissons Fossiles, 4to, 1843, i. p. 127. 



