PALEONTOLOGY 73 



short life. On the other hand the much higher 

 forms of Cidaris and Ceratodus have existed, the 

 former from the Permian, the latter from the 

 Triassic period. 



It follows from this that a species, or larger 

 group, once extinct, never reappears. No doubt 

 there are a few cases of long gaps between the re- 

 presentative of a form, but these are not more than 

 we should expect from the imperfection of the record. 

 One remarkable case is Ceratodus, a dipnoid fish 

 known by its remarkable teeth, which have been 

 found in rocks of Triassic and Jurassic age, and then 

 no more are known, except that living species occur 

 in the rivers of Queensland. But the teeth of 

 Jurassic age are very rare, and their subsequent 

 disappearance may be accounted for by the fish 

 having changed its habitat from the sea to rivers, 

 where there would be less chance of the teeth, which 

 are the only hard parts of the skeleton, being buried 

 and preserved. Indeed the reappearance of a form 

 in a locality from which it had long been absent is 

 no proof of the form having been extinct, for the 

 reappearance may be due to migration. For 

 example : no Trigonias lived in the northern hemi- 

 sphere in the Cainozoic era, but there were some in 

 Australia which still survive ; and if their descen- 

 dants were to increase and invade the northern 

 hemisphere, we should have there the appearance of 

 an extinct genus having come to life again. 



3. Life has been continuous on the earth ever 

 since its first appearance. This can be easily proved 

 if we assume that all the individuals of a species are 



