VIII.] HOMOLOGIES. 187 



is, that the segmentation of the skull its separation into 

 the three occipital, parietal, and frontal elements is most 

 complete and distinct in the highest class, and this can have 

 nothing, however remotely, to do with the cause suggested 

 by Mr. Spencer. 



Thus, then, there is something to be said in opposition 

 to both the aggregational and the mechanical explanations 

 of serial homology. The explanations suggested are very 

 ingenious, yet repose upon a very small basis of fact. Not 

 but that the process of vertebral segmentation may have 

 been sometimes assisted by the mechanical action sug- 

 gested. 



It remains now to consider what are the evidences in 

 support of the existence of an internal power, by the action 

 of which these homological manifestations are evolved. It 

 is here contended that there is good evidence of the exist- 

 ence of some such special internal power, and that not only 

 from facts of comparative anatomy, but also from those of 

 teratology 16 and pathology. These facts appear to show, 

 not only that there are homological internal relations, but 

 that they are so strong and energetic as to reassert and re' 

 exhibit themselves in creatures which, on the Darwinian 

 theory, are the descendants of others in which they were 

 much less marked. They are, in fact, sometimes even more 

 plain and distinct in animals of the highest types than in 

 inferior forms ; and, moreover, this deep-seated tendency 

 acts even in diseased and abnormal conditions. 



Mr. Darwin recognizes" these homological relations, 

 and does " not doubt that they may be mastered more or 

 less completely by Natural Selection." He does not, how- 

 ever, give any explanation of these phenomena other than 

 the imposition on them of the name " laws of correlation ; " 



16 " The Science of Abnormal Forms." 



17 " Animals and Plants under Domestication," vol. ii., p. 322 ; and 

 " Origin of Species," 5th edit, 1869, p. 178. 



