Only organic beings of certain classes can be preserved in a fossil condition, at least in any great number.

Many species when once formed never undergo any further change but become extinct without leaving modified descendants; and the periods during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retained the same form.

It is the dominant and widely ranging species which vary most frequently and vary most, and varieties are often at first local--both causes rendering the discovery of intermediate links in any one formation less likely.