THE GEOLOGICAL RECORD. 105 



the pig-like forms, or Suina, have not been very suc- 

 cessful ; the hippopotamus is nearing extinction, the 

 Tylopoda (camels) are but the isolated relics of a not 

 very flourishing division, and many allied families have 

 failed altogether. Thus we see that the existing families 

 of Ungulates are but the scattered remnants of a far 

 larger number of groups which flourished more or less 

 successfully in the past (Fig. 7). And the same may be 

 said of the history of all organisms, whether vertebrates 

 or invertebrates, whether animals or plants. Many are 

 the forms developed, few are those which survive. 

 Those who believe in a guiding force directing the course 

 of evolution must admit that it has been singularly blind 

 and inefficient, leading more often to destruction than to 

 success. 



Still, it is sometimes argued, organisms seem to get 

 into a groove of specialisation, to pursue a road along 

 which they can no longer stop, to become overspecialised 

 by virtue of some sort of momentum driving them over 

 the limits of usefulness to inevitable destruction. Thus 

 over and over again we see, in the record left by fossils, 

 animals acquiring a larger and larger size, and then sud- 

 denly dying out. The large Amphibia of the Carboni- 

 ferous, the monster Dinosaurs of the Jurassic and 

 Cretaceous, the gigantic Moas (flightless birds of New 

 Zealand), and among the mammals the huge Amblypods 

 and Titauotheres, the giant sloths, and others, are all 

 extinct. Again, some animals develop certain organs to 

 an excessive extent, as, for instance, the canine teeth in 

 the extinct sabre-toothed tiger (Machairodus), or the 

 monstrous antlers of the extinct Irish elk. Now it is 

 quite probable that these animals died out owing to over- 

 specialisation, a narrow adaptation to a particular en- 

 vironment accompanied by a corresponding loss of power 

 of accommodation to changed circumstances ; but it is a 

 mistake to assume without clear proof that the course of 

 their evolution can have been useless. Variation (p. 77) 



