CONTINUITY. 221 



in the course of transmission. This has long been my opinion, 

 and, as I understand, it is the view advocated by Mr. Herbert 

 Spencer in different parts of his valuable works.] 



The condition of the earth's surface, or at least of large 

 portions of it, has for long periods remained substantially the 

 same; this would involve a greater degree of fixity in the 

 organisms which have existed during such periods of little 

 change than in those which have come into being during 

 periods of more rapid transition ; for, though rejecting catas- 

 trophes as the modus agendi of nature, I am far from saying 

 that the march of physical changes has been always perfectly 

 uniform. 



There have been, doubtless, what may be termed secular 

 seasons, and there have been local changes of varying degrees 

 of extent and permanence ; from such causes the characteristics 

 of organised beings would be more concentrated in certain di- 

 rections than in others, the fixity of character being in the ratio 

 of the fixity of condition. This would throw natural forms into 

 certain groups which would be more prominent than others, 

 like the colours of the rainbow, which present certain pre- 

 dominant tints, though they merge into each other by 

 insensible gradations. 



While the evidence seems daily becoming stronger in 

 favour of a derivative hypothesis as applied to the succession 

 of organic beings, we are far removed from anything like a 

 sufficient number of facts to show that, at all events within 

 the existing geological periods capable of being investigated, 

 there has been always a progression from a simpler or more 

 embryonic to a more complex type. 



Professor Huxley, though inclined to the derivative hypo- 

 thesis, shows, in the concluding portion of his address to the 

 Geological Society, 1862, a great number of cases in which, 

 though there is abundant evidence of variation, there is none 

 of progression. There are, however, several groups of Verte- 

 brata in which the endoskeleton of the older presents a less 

 ossified condition than that of the younger genera. He cites 

 the Devonian Ganoids, the Mesozoic Lepidosteidse, the 

 Palaeozoic Sharks, and the more ancient Crocodilia and 

 Lacertilia, and particularly the Pycnodonts and Labyrintho- 



