538 ORIGIN OF MAMMALS xix. 3- 



a few very rare fossils, mostly lower jaw bones, give us some idea of 

 the nature of the animals that carried our type of organization through 

 the Jurassic period. Then from the top of the Cretaceous, about 70 

 million years ago, come the first fossil remains of mammals similar to 

 those alive today; rare, insectivorous creatures, not widely different 

 from modern hedgehogs. This was the time of the beginning of an 

 astonishing revolution, which completely altered the life on the face 

 of the earth. The descendants of these shrew-like animals multiplied 

 exceedingly in the new conditions, and by the earliest Palaeocene 

 times, 70 million years ago, they had produced recognizable ancestors 

 of most of the modern orders of mammals. 



Our knowledge about this revolution is still very dim. Some people 

 claim that it accompanied one of the major geological crises, a 

 period in which the surface of much of the earth became unstable, 

 there were great volcanic outflows and the building of vast mountain 

 chains. It is possible that such upheavals led to the disappearance of 

 the swampy conditions, which had been so suitable for large reptiles, 

 and to the appearance of dry uplands on which the mammals and birds 

 flourished, especially where the climate was cold. We must be careful 

 here, however, not to argue in a circle. Our evidence about the climate 

 is derived from the changes in the populations, as revealed by the 

 fossils. Study of the plant remains, however, confirms the supposition 

 that conditions became drier during this time. 



Our knowledge of the origins of mammals derived from fossils is 

 supplemented by certain surviving mammals, whose structure shows 

 them to have diverged rather early from the main stock, especially the 

 monotremes (duck-billed platypus and Echidna) and the marsupials 

 (kangaroos, opossums, &c). Unfortunately it is still not clear exactly 

 how these survivors are related to the main stocks as revealed by the 

 fossils. The monotremes are unknown except as Pleistocene fossils; 

 tantalizingly enough we know only little about the affinities of this 

 ancient group. The characteristics of their bony skeleton show that 

 they must have diverged from other mammals well back in Mesozoic 

 times. For instance, the pelvic and pectoral girdles are very 'reptilian' 

 in structure. It is therefore not wild speculation to use the characters 

 of the soft parts of monotremes to deduce those of the mammalian 

 stock in late Triassic or early Jurassic times, say, 180 million years 

 ago. The marsupials appear as fossils in the late Cretaceous. Some of 

 the earliest forms are very like modern opossums, and we may con- 

 clude that the condition of modern marsupials throws some light on 

 the probable condition of the soft parts of mammals 70 or 80 million 



