xxxii. 6 RATE OF CHANGE 771 



range of modern primates. The rate of evolution of horses and chalico- 

 theres has been about the same (0-13 genera per million years) and 

 much faster than that of ammonites (0-05), assuming that 'genus' has 

 a similar meaning in the two cases. 



Using taxonomic rates it has been shown that many vertebrate 

 major groups seem to evolve fast at their 'first appearance'. This rapid 

 evolution (tachytely) is presumably the result of moving into a new 

 adaptive zone, which we notice ex post facto as the beginning of a 

 higher taxonomic group. The chances of finding these transition types 

 as fossils may be unduly small if evolution is rapid and especially if it 

 occurs in a small population (or a large one divided into small units; 



5. Wright). Claims to have found the 'centre of origin' of a major 

 group must therefore be looked upon with suspicion. In any case, 

 parallel evolution may carry several lines over the arbitrary line we use 

 to mark a higher taxonomic order; at least five lines of therapsid 

 reptiles crossed to become mammals (p. 545). 



This discussion may make it seem unreal to speak of 'origins' of 

 higher taxonomic groups. Indeed, it is probably misleading to look 

 for major 'branches' in what must be a multiple evolutionary 'bush'. 

 Nevertheless there is evidence that rate of change is not constant. 

 Apart from such examples as those already discussed, there are many 

 others. Bats, as Simpson points out, have certainly evolved more 

 slowly since they first got wings and 'broke through' to a new en- 

 vironment than in the period of that change itself. 



6. Vertebrates that have evolved slowly 



We may accept then the concept of bursts of rapid evolution, 

 followed by slower change. In many lines after the rapid change there 

 is a period over which many genera become extinct. However, a few 

 linger on for times longer than would be expected (bradytely). This 

 general pattern can be seen for fishes in Fig. 5 13^. This phenomenon 

 of bradytely produces phylogenetic relicts, of which there seem to be so 

 many that some general explanation of them is desirable. Neoceratodus 

 has a good claim to be considered the 'oldest' living vertebrate; it is 

 very similar to fossils found in the Triassic, nearly 200 million years 

 ago. Even in this case, however, there have been slight changes and 

 the Triassic form is placed in a distinct genus *Ceratodiis. Latimeria 

 provides us with an example of survival with little change for nearly 

 100 million years, as well as the humbling thought that no fossil 

 relatives are known throughout that time. Heterodontus, the Port 

 Jackson shark, is another very ancient fish; it is closely similar to 



