EVOLUTION 



713 



made on the occurrence in different rocks of similar fossils. 

 Changes in the two sets of fossils cannot subsequently be legiti- 

 mately used as evidence for evolution, although they may be 

 built up into a story which is not incompatible with it. 



In addition to the relatively 

 small number of lineages which 

 have been worked out, there is 

 general evidence that the simpler 

 or structurallv less advanced 

 groups appeared earlier than the 

 more complex ones. Fish occurred 

 before Amphibia, these before 

 reptiles, and reptiles before birds 

 and mammals. Occasionally ' miss- 

 ing links ' are found, isolated 

 fossils which partake of the 

 nature of two groups and stand between them in time. ArchcB- 

 opteryx, for example, of the Jurassic, is a bird-like animal with 



Fig. 548. — Extinct Crossopterygian 

 Fislies. — From Swinnerton. 



(a; Noioptychiiis ; (b) Glyptopomus. Note the 

 stout central portions of the fins, con- 

 taining skeleton and doubtless muscle, and 

 thus having the makings of legs. 



unpaired dorsal fins 



spiracle 



diphycercal 

 tail 



gular 



pectoral fin 



pelvic fin 



unpaired 

 anal tin 



Fig. 549. — A living Crossopterygian, Latimeria chalumncB. — From Thorne. 

 Drawn after a photograph by J. L. B. Smith. .},-, natural size. 



teeth and a reptilian tail (Fig. 330). Tritylodon, a small creature 

 from the Triassic, was formerly regarded as the earliest of true 

 mammals, but is now considered to be a reptile on account of 

 its jaw articulation ; it may reasonably be considered to be 

 something between the two, and without a knowledge of its whole 

 structure and mode of life, which we are never likely to have, it 

 does not much matter which we call it. Within the ph^da, then, 

 palaeontology strongly suggests evolution. For the origins of 

 the phyla, it gives no evidence one way or the other. 



