IMPRESSIONS OF THE PAST 35 



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. 



Worm burrows, to be sure, are hardly foot- 

 prints, 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 favorable, 

 ever since animals began to walk. All that 

 was needed was a medium in which impres- 

 sions 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 



* Within the last Jew years rvhat are believed to be indica- 

 tions 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. 



