1.   A pelagic habit and a delicate, gelatinous body account for their poor fossil record.

2.   But the fossil record of the insects is far from perfect - very sporadic and selective.

3.   Crocodiles and lizards have a fossil record extending back to the Triassic, snakes to the Cretaceous.

4.   Marine sponges have a long fossil record.

5.   Most are microscopic, and many have no skeleton and therefore lack a fossil record.

6.   The fossil record bears this out.

7.   The fossil record is silent on this transition, but the earlier stages of mollusc evolution are becoming clear.

8.   The greatest problems in the fossil record, however, are the sudden extinctions.

9.   The growth of the human embryo recapitulated the history of animal life as revealed by the fossil record.

10.   We also know that the fossil record is fragmentary in the extreme.