

1880.] ON THE ARRANGEMENT OP THE MAMMALIA. 



649 



1. On the Application of the Laws of Evolution to the 

 Arrangement of the Vertebrata, and more particularly 

 of the Mammalia. By T. H. Huxley, F.R.S. 



[Keceived Norember 23, 1880.] 



whTcirfn mtfuTn; "r '''"' f "'^'^'^ ""'' "«^ '^^^^ disputed, and 

 m.ncement of the lert.aiy epoch and the present time, the eroun of 

 oWe^i^'^f '"1 .''f "/^presented by a series of forms of wh cl7the 

 oldest IS that which departs least from the general type of structure 



differ: S uXT"^' '-''' ''' '''-'' ''''^' ^ ^^^ 



snbemtf :if •? '^'^T^ ^''""7 ''i"^"" ^°""^' Possesses four complete 

 subequal digits on the tore foot, three on the hind foot • the \u a s 

 complete and distinct from the radius • the fibnl . i = V i , ? 

 distinct from the tibia- tliere -ire 44 teVfl, L r n f^'^P^^^e and 

 beinff nresenf «nrl tL I ^ f ., , ' ^''^ ^"'' number ot canines 



oeing present, and the cheek-teeth having sliort crowns witli simnle 

 pa terns and early formed roots. The latest, on the o her and has 

 only one complete digit on each foot, tlie others bein. rep e S^ 



fcrus^'thrfiM ""^V^,/^" "'"^ i^ '•''^'^' and^a.?k Sd wi J 

 ^hb fh. f-K ?. '^ '• '*''^ "^"""^ "'^'^"'^^'^ ""d partially ankvlosed 

 pr ssed in ?ie'r ,''""r 'T'^ ''' ^'''''''^^y '^^ completely' sup- 

 vJZl {""^^f ' ^^'' ^''^ cheek-teeth usually remain unde- 



fiTi' "i'^ ''^''" '^''y ^PP^'-^^ ^'^ ^''y S'"'-^" ; thi othe cheek- 

 : med7o:tr"^rF" t' )'^'''' eo4hcated' patterns Id lit- 

 chaTacters ^ intermediate age exhibit intermediate 



With respect to the interpretation of these fa6ts, two hypotheses 



icceste fo;mr':r '° '^ ""-^'"f 'l* ''''' «"^ ^^ '"'^ the- 

 successi e foims of equme animals have come into existence inde- 

 pendently of one another. The other assumes that^hev ar th^ 

 result ot the gradual modification undergone by the suc^essh-e 

 members of a continuous line of ancestry J' "'« successne 



thetis \7nZ\ 7r n^f ^"^ '°''°S*^' "'^^•^'^•'^^ the first hypo- 

 thesis I do not feel called upon to discuss it. The adoption of tbe 



"SulTiSa: r"'^'^^ ^° the acceptance of thTS. -ne' 

 e^oiutionj so iar as Horses are concerned ; and, in the abseurn of 

 evidence to the contrary, I shall suppose that it is accepted 



llms, since the commencement of the Eocene epoch, the animals 



^^ssryjhi^n;!;;^.^ ^'^''- '- -'-^-^ p--- 



relado^tTotlSs.'"" " """^ °' development of some parts iu 



2. Certain parts have uP.dergone complete or partial suppression 



3. Certain parts, which were originally distinct, have coafesced 

 Employing the term "law" simply in the sense era Teneml 



statement of fact, ascertained by obse'rv'ation, I shall spe k of h s 

 Proc. Zool. SOC.-I880, No. XLIII. 43 



