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 few years what are believed to be indica- 
tions of bacteria have been described from carboniferous rocks. 
Naturally such announcements must be accepted mith 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. 
