EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 127 



Thus the correctness of the theory that the skull is formed of 

 modified vertebrae in reality depends on the special standpoint from 

 which we regard the name " vertebra." Viewed as to its development, 

 and compared with the development of vertebrae, the segments 

 which every anatomist recognises in the skull assuredly present no 

 resemblance to the joints of the backbone. But if we enlarge the 

 definition of a vertebra to include the idea of a segment of the 

 skeleton forming the axis of the body and protecting the nervous and 

 blood centres, then the segments of the skull may correspond to 

 such description. Here, however, we construct a definition of the 

 vertebra, without reference to its development ; the latter source of 

 information being the most trustworthy in reference to the nature 

 of the things and belongings of life. As Huxley has remarked 

 concerning skull and spine, "though they are identical in general 

 plan of construction, the two begin to diverge as soon as the one puts 

 on the special character of a skull, and the other that of a vertebral 

 column ; the skull is no more a modified vertebral column than 

 the vertebral column is a modified skull." This view exactly 

 accords with the requirements of the theory of evolution, which 

 would impress that, in the course of descent from the primitive 

 spinal and skulless stage of organisation, the skull has been 

 specialised from the general vertebrate type, just as the vertebrae 

 themselves have risen from their first rude outlines to their present 

 and modified condition. 



Thus have grown the ideas which the casual study of a broken 

 sheep's skull first generated ; and thus do we find an illustration of 

 the method in which a study of homology leads us towards an 

 understanding of the true nature of an organ or part in living 

 beings. But for this science of likeness but for the results of long, 

 careful, and laborious research into the comparisons which may be 

 legitimately drawn between the formation of the skull in one animal 

 and in another the answer to the question " What is a skull ? " might 

 have been left in the position of a riddle propounded by the Sphinx 

 itself. Thus much has resulted from the study of likenesses 

 namely, a clear gain of much knowledge concerning the true nature 

 of an intricate portion of the animal frame. It yet remains to be 

 shown how the progress of evolution has helped and aided the true 

 understanding of the modifications which the skull has undergone in 

 its progress from the unspecialised type of primitive vertebrate life; and, 

 conversely, how the existence of such modifications aids, confirms, and 

 supports the basis on which the development theory may be said to rest. 

 Says Professor Parker, "We are necessarily led to see that this unity of 

 structure, this relationship, includes extinct creatures as well as those 

 now living. And the student cannot but seek for some further light 

 than is involved in the establishment of the fact that there is a 



