Royal Institution. -69 



ROYAL INSTITUTION OF GEEAT BRITAIN. 



April 20, 1855.— William Robert Grove, Esq., M.A., Q.C., F.R.S., 



Vice-President, in the Chair. 



On certain Zoological Arguments commonly adduced in favour of 

 the hypothesis of the Progressive Development of Animal Life 

 in Time. By T. H. Huxley, Esq., F.R.S. 



When the fact that fossilized animal forms are no lusus naturcSy 

 but are truly the remains of ancient living worlds, was once fully 

 admitted, it became a highly interesting problem to determine what 

 relation these ancient forms of life bore to those now in existence. 



The general result of inquiries made in this direction is, that the 

 further we go back in time, the more different are the forms of life 

 from those which now inhabit the globe, though this rule is by no 

 means without exceptions. Admitting the difference, however, the 

 next question is, what is its amount ? Now it appears, that while 

 the Palaeozoic species are probably always distinct from the modern, 

 and the genera are very commonly so, the orders are but rarely dif- 

 ferent, and the great classes and sub-kingdoms never. In all past 

 time we find no animal about whose proper sub-kingdom, whether 

 that of the Protozoa, Radiata, Annulosa, Mollusca, and Vertehratay 

 there can be the slightest doubt ; and these great divisions are those 

 which we have represented at the present day. 



In the same way, if we consider the Classes, e. g. Mammalia, Aves, 

 Insecta, Cephalopoda, Actinozoa, &c., we find absolutely no remains 

 which lead us to establish a class type distinct from those now 

 existing, and it is only when we descend to groups having the rank 

 of Orders that we meet with types which no longer possess any living 

 representatives. It is curious to remark again, that, notwithstanding 

 the enormous lapse of time of which we possess authentic records, 

 the extinct ordinal types are exceedingly few, and more than half of 

 them belong to the same class — Reptilia. 



The extinct ordinal Reptilian types are those of the Pachypoda, 

 Pterodactyla, Enaliosaurea, and Labyrinthodonta -, nor are we at 

 present acquainted with any other extinct order of Vertebrata. 

 Among the Annulosa (including in this division the Echinodermata) 

 we find two extinct ordinal types only, the Trilobita and the Cys^ 

 tidece. 



Among the Mollusca there is absolutely no extinct ordinal type ; 

 nor among the Radiata {Actinozoa and Hydrozoa) ; nor is there 

 any among the Protozoa. 



The naturalist who takes a wide view of fossil forms, in connec- 

 tion with existing life, can hardly recognize in these results anything 

 but strong evidence in favour of the belief that a general uniformity 

 has prevailed among the operations of Nature, through all time of 

 which we have any record. 



Nevertheless, whatever the amount of the difference, and however 

 one may be inclined to estimate its value, there is no doubt that the 



