82 Mr. Huxley on the Progressive [April 20, 



WEEKLY EVENING MEETING, 



Friday, April 20. 



William Robert Grove, Esq. M.A. Q.C. F.R.S. 

 Vice-President, in the Chair. 



T. H. Huxley, Esq. F.R,S. 



On certain Zoological Arguments commonly adduced in favour of 

 the hypothesis of tJie Progressive Development of Animal Life 

 in Time, 



When tlie fact that fossilized animal forms are no lusus natur<2, 

 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 ex- 

 istence. 



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 different, 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 

 Vertebraia, there can be the slightest doubt ; and these great divi- 

 sions are those which we have represented at the present day. 



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

 hisecta, 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, notwith- 

 standing 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 — JReptilia. 



The extinct ordinal Reptilian types are those of the Pachypoda, 

 Pterodactyla, Enaliosauria, and Labyrinthodonta ; nor are we 

 at present acquainted with any other extinct order of Vertebrata. 

 Among the Annulosa (including in this division the Echinoder- 



