28G THE WOKLD OF LIFE 



as soon as the race they represent has reached its prime and begins 

 to be on the down grade. Among famiUar instances may be men- 

 tioned the curiously spiny Graptolites at the end of the Silurian, the 

 horned Pariasaurians at the beginning of the Trias, the armour- 

 plated and horned Dinosaurs at the end of the Cretaceous, and the 

 cattle or deer of modern Tertiary times. . . . The growth of 

 these excrescences, both in relative size and complication, was con- 

 tinual and persistent until the climax was reached and the extreme 

 forms died out. ... 



" It appears, indeed, that when some part of an animal (whether 

 an excrescence or a normal structure) began to grow relatively 

 large in successive generations during geological time, it often ac- 

 quired some mysterious impetus by which it continued to increase 

 long after it had reached the serviceable limit. The unwieldy 

 antlers of the extinct Sedgwick's deer and Irish deer (Fig. 95), 

 for example, must have been impediments rather than useful 

 weapons. Tlie excessive enlargement of the upper canine teeth in 

 the sabre-toothed tigers (Machaerodus and its allies) must also 

 eventually have hindered rather than aided the capture and eating 

 of prey.'' ^ 



Dr. Woodward further remarks: 



" The curious gradual elongation of the face in the Oligocene 

 and Miocene Mastodons can only be regarded as another illustra- 

 tion of the same phenomenon. In successive generations of these 

 animals the limbs seem to have grown continually longer, while the 

 neck remained short, so that the head necessarily became more and 

 more elongated to crop the vegetation on the ground. A limit of 

 mechanical efficiency was eventually reached, and then there sur- 

 vived only those members of the group in which the attenuated 

 mandibles became shortened, leaving the modified face to act as a 



1 The species Maclicerodus neogceus, the skull of which is shewn in Fig. 94, 

 appears lo have had the largest canines of any species of the genus; and we 

 are told by Messrs. Xicholson and Lydekker (Manual of Palaeontology^ ii. p. 

 1449) that the upper carnassial tooth (the fourth premolar) "has four 

 distinct lobes, and is thus the most complex example of this type of tooth 

 known." The canines were about 9 inches long (more than half the length 

 of the whole skull), and very massive in proportion. It became extinct in 

 South America in the Pleistocene period, about the same time as the last of 

 the European species. 



