306 THE ORIGIN OF SPECIES 



forms of life help to fill 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 con- 

 fine 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 generalized 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 dis- 

 tinguished 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 Ruminants 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 South Amer- 

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

 will deny that the Hipparion is intermediate between the existing 

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

 necting link in the chain of mammals is the Typotherium from 

 South America, as the name given to it by Professor Gervais ex- 

 presses, and which cannot be placed in any existing order. The 

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

 the most remarkable peculiarities in existing dugong and lamen- 

 tin is the entire absence of hind limbs, without even a rudiment 

 being left; but the extinct Halitherium had, according to Profes- 

 sor 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, 

 which have been placed by some naturalists in an order by them- 

 selves, are considered by Professor Huxley to be undoubtedly 

 cetaceans, "and to constitute connecting links with the aquatic 

 carnivora." 



Even the wide interval between birds and reptiles has been 

 shown by the naturalist just quoted to be partially bridged over 



