GENERAL SUCCESSION OF ORGANIC TYPES. 



ever, is a matter of opinion, and reasons are not wanting for 

 regarding them as the highest of their class. 



It only remains to add that nothing further is contended 

 for here than the general fact of there having been a pro- 

 gression of morphological types, the lowest presenting them- 

 selves first, the highest being the last to appear upon the 

 scene. It is by no means contended that the Ganoid Fishes 

 of the Upper Silurian rocks were in any way degraded 

 members of their ord^r, or inferior in point of organisation 

 to the Ganoids of the present day. On the contrary, there 

 is reason to think that many types early presented a de- 

 velopment more varied than that exhibited by their suc- 

 cessors. It is simply contended that, on the whole, there has 

 been a zoological progression as we ascend from the Cambrian 

 period to the present day. It is also to be remembered, 

 that though the commencement of the Invertebrate sub- 

 kingdoms may be unknown to us, a similar progression can 

 be in many cases shown as regards the orders and classes of 

 these, even more completely than in the case of the Verte- 

 brate sub-kingdom. 



Lastly, the evidence of Palaeontology points, in the main, 

 to the operation of some general law of evolution, whereby 

 the later forms of life have been derived from the older ones. 

 That this law has acted along with, and has sometimes been 

 counteracted by, some other and as yet obscure law regu- 

 lating the appearance of new types, seems equally certain ; 

 but it is not necessary to enter upon this complex and im- 

 perfectly understood question in this place. We are dealing 

 here primarily with facts, and in the following pages we 

 shall meet with unmistakable evidence of the operation of 

 some law of evolution, while we shall, at the same time, find 

 ourselves confronted with phenomena which, in the present 

 state of our knowledge, appear to be irreconcilable with the 

 universal or exclusive action of this law. It would be an easy 

 solution of the difficulty to adopt the course of definitely 

 accepting some hard-and-fast theory upon this subject, and 

 to bring forward prominently all the known facts favouring 

 this theory, while we left in comparative abeyance the facts 

 pointing in other directions, or explained them away by 



