ISOLATION 



*3 l 



In this inquiry we are obliged to depend on the relatively 

 few groups which both provide suitable material and have 

 been subjected to sufficient taxonomic study. We are not 

 so much concerned with the maximum as with the minimum 

 time which such a change may take. We can never know 

 whether a fossil form which is identical in structure with a 

 modern one would, in fact, be able to interbreed with it. 

 But even in the majority of living species we do not know 

 whether interbreeding is possible, and we are endeavouring 

 rather to estimate something which has a meaning in present- 

 day taxonomy, viz. how long it has taken to evolve differences 

 which would be considered sufficient to separate races or 

 species, if they characterised recent forms. 



Modern species known to have persisted since pre-Tertiary 

 times are rare. An interesting example is the shark Scapanor- 

 rhynchus owsteni, which was first described from fossil teeth in 



Fig. i 6. — Scapanorrhynchus owsteni. 

 (From Norman, 1931.) 



the Upper Cretaceous but has since been found living off 

 the coast of Japan (Norman, 1931, p. 124). In other 

 instances, as perhaps in the Brachiopoda, the characters 

 available for study in the fossil state are so few that the com- 

 parison with recent species could not be expected to be very 

 enlightening. But it appears that, just as some species with 

 discontinuous range soon form numerous races while others 

 remain relatively homogeneous, so the rate of evolution, 

 judged by palaeontological evidence, must be variable from 

 group to group, and probably depends on innate potentialities. 

 Wheeler (191 3, chapter x) has discussed the fossil history 

 of the ants. Many of the amber fossils are perfectly preserved 

 and are as capable of exact study as recent specimens. In the 

 Sicilian Amber (Lower Oligocene) nearly 69 per cent, of the 

 genera are still living. Three species, belonging to different 

 genera, are not separable from well-known living forms. There 



