24 ANIMALS OF THE PAST 



filled with fine mud, and while this interpretation may 

 be wrong there is, on the other hand, no reason why it 

 may not be correct. Plant and animal life must have 

 had very lowly beginnings, and it is not at all probable 

 that we shall find any trace of the simple and minute 

 forms with which they started, ^ though we should not be 

 surprised at finding hints of the presence of living 

 creatures below the strata in which their remains are 

 actually known to occur, and in the Middle Cambrian 

 Dr. Walcott has discovered an abundant fauna of 

 worms, brachiopods, trilobites and other crustaceans. 



Worm burrows, to be sure, are hardly footprints, but 

 tracks are found in Cambrian rocks just above the 

 strata in which the supposed burrows occur, and from 

 that time onward there are tracks a-plenty, for they 

 have been made, wherever the conditions were favor- 

 able, ever since animals began to walk. All that was 

 needed was a medium in which impressions could be 

 made and so filled that there was imperfect adhesion 

 between mould and matrix. Thus we find them formed 

 not only by the sea-shore, in sands alternately dry and 

 covered, but by the river-side, in shallow water, or even 

 on land where tracks might be left in soft or moist earth 

 into which wind-driven dust or sand might lodge, or 

 sand or mud be swept by the mimic flood caused by a 

 thunder shower. 



So there are tracks in strata of every age; at first 

 those of invertebrates: after the worm burrows the 



'Within the last few years what are believed to be indications of 

 bacteria have been described from carboniferous rocks. Naturally 

 such announcements must be accepted with great caution, for while there 

 is no reason why this may not be true, it is much more probable that 

 definite evidence of the effects of bacteria on plants should be found than 

 that these simple, single-celled organisms should themselves have been 

 detected. 



