496 THE AMERICAN NATURALIST. [VoL. XXXIV. 
furnish valuable data in solving problems of descent and ex- 
plain to the biologist certain obscure points in the struc- 
ture of living forms. But they afford evidence of another 
kind, the value of which is determined by the ability of the 
observer to make a correct interpretation. They may reveal 
much concerning the habits of species, whether terrestrial or 
aquatic, and their habitat ; they may give clues to temperature 
and other details upon which climate depends; they may indi- 
cate, if the species be aquatic, the nature of the water, whether 
fresh, salt, or brackish, whether shallow or deep, whether cold 
or warm, etc. It is from his knowledge of existing conditions 
of life and growth that the observer is able to interpret the 
conditions attendant upon certain characteristic growth of the 
past : that, for example, bright, clear skies, and pure sea-water, 
of not too great depth and free from the chilling influence of 
cold currents, are conducive to a profusion of coral growth ; 
that alternate exposure to sea and air is satisfactory to mus- 
sels and other littoral species ; that clams thrive best in the 
sand and mud banks exposed between the tides. Knowledge 
of this kind enables the acute observer to gain a certain 
amount of %istorical insight, when kindred forms are found in 
strata, concerning the conditions that have prevailed upon the 
earth in remote times of which there is left no direct record. 
It must be admitted, also, that a correct interpretation 
involves much evidence other than that afforded by organic 
remains. The rocks themselves, irrespective of their fossil 
contents, afford most valuable data respecting the conditions 
attendant upon their deposition: mud, for instance, indicating 
turbulent times, erosion and waste of land surfaces, heavy 
rainfalls, swollen rivers, and flooding; foraminiferal ooze 
and other fine-grained calcareous matter, deep-sea deposits ; 
sand and gravel, beach deposits. Data of this kind are not to 
be ignored ; nor are those changes, such as the thickening and 
thinning of strata, the passage of limestone beds into shale or: 
sandstone, or the reverse, which may indicate that the deposi- 
tion had taken place near to or far from land masses. The 
information thus derived is to be woven into the record ; it is 
essential for a complete interpretation. 
