CHAP. XI.] AFFINITIES OF EXTINCT SPECIES. 283 



On the Affinities of Extinct Species to each other, and to 

 Living Forms. 



Let us now look to the mutual affinities of extinct and living 

 species. All fall into a few grand classes; and this fact is at once 

 explained on the principle of descent. The more ancient any form 

 is, the more, as a general rule, it differs from living forms. But, 

 as Buckland long ago remarked, extinct species can all be classed 

 either in still existing groups, or between them. That the extinct 

 forms of life help to till up the intervals between existing genera, 

 families, and orders, is certainly true ; but as this statement has 

 often been ignored or even denied, it may be well to make some 

 remarks on this subject, and to give some instances. If we confine 

 our attention either to the living or to the extinct species of the 

 same class, the series is far less perfect than if we combine both 

 into one general system. In the writings of Professor Owen we 

 continually meet with the expression of generalised forms, as 

 applied to extinct animals; and in the writings of Agassiz, of 

 prophetic or synthetic types; and these terms imply that such 

 forms are in fact intermediate or connecting links. Another 

 distinguished palaeontologist, M. Gaudry, has shown in the most 

 striking manner that many of the fossil mammals discovered by 

 him in Attica serve to break down the intervals between existing 

 genera. Cuvier ranked the lluminants and Pachyderms, as two 

 of the most distinct orders of mammals : but so many fossil links 

 have been disentombed that Owen has had to alter the whole 

 classification, and has placed certain pachyderms in the same sub- 

 order with ruminants ; for example, he dissolves by gradations the 

 apparently wide interval between the pig and the camel. The 

 Ungulata or hoofed quadrupeds are now divided into the even- 

 toed or odd-toed divisions ; but the Macrauchenia of S. America 

 connects to a certain extent these two grand divisions. No one 

 will deny that the Hipparion is intermediate between the existing 

 horse and certain older ungulate forms. What a wonderful con- 

 necting link in the chain of mammals is the Typotherium from 

 S. America, as the name given to it by Professor Gervais expresses, 

 and which cannot be placed in any existing order. The Sirenia 

 form a very distinct group of mammals, and one of the most 

 remarkable peculiarities in the existing dugong and lamentin is 

 the entire absence of hind limbs without even a rudiment being 

 left ; but the extinct Halitherium had, according to Professor 

 Flower, an ossified thigh-bone "articulated to a well-defined 

 acetabulum in the pelvis," and it thus makes some approach to 

 ordinary hoofed quadrupeds, to which the Sirenia are in other 

 respects allied. The cetaceans or whales are widely different from 

 all other mammals, but the tertiary Zeuglodon and Squalodon, 



