EXTENSIONS OF DARWINISM 265 



type, but that both arose from an early, more generalised 

 type, he says : 



" To have proved, for example, that flying reptiles did not pass 

 into birds or bats, that hoofed Dinosaurs did not change into 

 hoofed mammals, and that Ichthyosaurs did not become porpoises, 

 and to have shown that all these later animals were mere mimics 

 of their predecessors, originating independently from a higher yet 

 generalised stock, is a remarkable achievement." 



Then comes a reference to the subject we are now 

 discussing : 



" Still more significant, however, is the discovery, that towards 

 the end of their career through geological time, totally different 

 races of animals repeatedly exhibit certain peculiar features which 

 can only be described as infallible marks of old age. The growth 

 to a very large size is one of these marks, as we observe in the 

 giant Pterodactyls of the Cretaceous period, the colossal Dinosaurs 

 of the Upper Jurassic and Cretaceous, and the large mammals of 

 the Pleistocene and the present day. It is not, of course, all the 

 members of a race that increase in size ; some remain small until 

 the end, and they generally survive long after the others are extinct. 



" Another frequent mark of old age in races was first discussed 

 and clearly pointed out by Professor C. E. Beecher of Yale. It is the 

 tendency of all animals with skeletons to produce a superfluity of 

 dead matter, which accumulates in the form of spines or bosses as 

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

 on the down grade. Among familiar instances may be mentioned 

 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 continual 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 acquired 

 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. The 

 excessive enlargement of the upper canine teeth in the sabre-toothed 

 tigers (Machserodus and its allies) must also eventually have hindered 

 rather than aided the capture and eating of prey." x 



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

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



