Many of the problems that we meet in evolutionary argumentation arise only because animals are inconsiderate enough to evolve at different rates, and might even be inconsiderate enough not to evolve at all. If there were a law of nature dictating that quantity of evolutionary change must always be obligingly proportional to elapsed time, degree of resemblance would faithfully reflect closeness of cousinship. In the real world, however, we have to contend with evolutionary sprinters like birds, who leave their reptile origins standing in the Mesozoic dust - helped, in our perception of their uniqueness, by the happenstance that their neighbours in the evolutionary tree were all killed by a celestial catastrophe. At the other extreme, we have to contend with 'living fossils' like Lingula which, in extreme cases, have changed so little that they might almost interbreed with their remote ancestors, if only a matchmaking time-machine could procure them a date.
Lingula is not the only famous example of a living fossil. Others include Limulus, the horseshoe 'crab', and coelacanths, which we shall meet in the next chapter.
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