824 LIFE : OUTLINES OF GENERAL BIOLOGY 



veritable flying dragons, as instance of a remarkable order which 

 has had neither continuance nor successors. Similarly, some of the 

 old orders of mammals, such as the huge hoofed Amblypods of the 

 Eocene, seem to have disappeared without leaving any near 

 relatives, while many comparatively primitive mammalian types 

 still go on. 



What can be said in regard to the causes of extinction? In the 

 first place there is little to suggest that sudden extinction has 

 often occurred, save in recent times, when man has intervened 

 unconsciously or ruthlessly. Thus the European Bison has ceased to 

 be a wild animal since the Great War; the till lately multitudinous 

 Passenger Pigeon disappeared in a very short time, even within 

 living memory ; but the rock record of race extinctions points almost 

 always to a gradual waning. Local floods, earthquakes, volcanic 

 eruptions, landslips, and the like may have rapidly exterminated 

 species and even genera; herbs and trees alike seem peculiarly 

 liable to destruction by mould or insect enemies; yet on the whole 

 races seem to die out but slowly. Three suggestions may be offered. 

 (a) In many cases the waning away of an order, or even of a class 

 of animals, may have been associated with the appearance of some 

 formidable new competitor; thus cuttlefishes would tend to exter- 

 minate Trilobites, and Ichthyosaurs in their turn would thin out 

 the cuttlefishes, whose beaks often crowd their stomachs, {b) At this 

 day many highly specialised types are unable to adjust themselves 

 to what seem to us slight changes in their environment, and this may 

 have been even more true amid the drastic changes of the past. 

 Thus Marsh has suggested that some of the magnificently built 

 extinct reptiles might have for their epitaph: "I and my race died 

 of over-specialisation." (c) Do not a good many extinct types seem 

 to have fallen victims to their own constitutions? Witness numerous 

 reptiles, surely overgrown in body beyond their brains, like the 

 colossal Diplodocus carnegii, which even in cast is so striking an 

 attraction of the British Museum, and some Mammals also, huge 

 and sluggish, like Megatherium, and even too calcareous, like the 

 Glyptodon, wellnigh stone-coffined — all, in short, in some way too 

 extreme. Among backboned animals, endocrinal glands may have 

 broken down before some difficult problem of bodily regulation, or 

 may by excess of function have hurried the creature along some 

 fatal path. In gigantism and in nanism, both difficulties may operate 

 in varied proportion, for such cases recall the vivid phrase of 

 Lucretius, of animals "hampered in their own death-bringing 

 shackles". 



Persistent Types. — In contrast to the Becoming and Disap- 

 pearing {Werden and Vergehen), which is characteristic of the long- 

 drawn-out history of living creatures, and in peculiar contrast to 

 the organic flux which is exhibited to-day in certain well-known 



