It is animals and plants which lived in or near water whose remains are most likely to be preserved, for one of the necessary conditions of preservation is quick burial, and it is only in the seas and rivers, and sometimes lakes, where mud and silt have been continuously deposited, that bodies and the like can be rapidly covered over and preserved. ...
likely=possibly
deposited=saved=piled up 堆积
mud (from the seashore)
the man and the like: 就他那样的人（否定意义）
cover over: 全部覆盖
第一段：两个强调句型分别加: 1. 加原因状语 2. 加地点状语
接下来课文...But even in the most favourable circumstances only a small fraction of the creatures that die are preserved in this way before decay sets in or, even more likely, before scavengers eat them. ...