THE PROGRESS OF DEGRADATION. 159 



of all those Silurian placoids yet known, of whicli the family 

 can be determined, or anything with safety predicated, the 

 cervical division is found to occur as a series of vertebrae : 

 they present in this, as in the hinder portion of their bodies, 

 the homological symmetry of organization typical of that ver- 

 tebral subkingdom to which they belong. 



In the second great period of ichthyic existence, — that of 

 the Old Bed Sandstone, — we find the first example, in the 

 class of fishes, of " monstrosity through displacement of parts," 

 and apparently also — in at least two genera, though the evi- 

 dence on this head be not yet quite complete — of " monstro- 

 sity through defect of parts." In all the ganoids of the pe- 

 riod, with (so far as we can determine the point) only two 

 exceptions, the scapular bases of the fore limbs are brought 

 forward from their typical place opposite the base of the cer- 

 vical vertebra, and stuck on to the occipital plate. There 

 occurs, in consequence, in one great order of the ichthyic 

 class, such a departure from the symmetrical type as would 

 take place in a monster example of the human family in whom 

 the neck had been annihilated, and the arms stuck on to the 

 back of the head. And in the genera Coccosteus and Fterich- 

 thys we find the first example of degradation through defect. 

 In the Pterichthys the hinder limbs seem wanting ; and in 

 the Coccosteus we find no trace of the fore limbs. The one 

 resembles a monster of the human family born without hands ; 

 the other a monster born without feet. Ages and centu- 

 ries pass, and long unreckoned periods come to a close ; 

 and then, after the termination of the Palaeozoic period, we 

 see that change taking place in the form of the ichthyic tail 

 to which I have already referred (and to which I must refer 

 at least once more), as singularly illustrative of the progress 

 of degradation. Yet other ages and centuries pass away, 

 during which the reptile class attains to its fullest develop- 

 ment^ in point of size, organization, and numbers ; and then, 



