Whilst it seems certain, therefore, that many of
the phenomena of the succession of animal life in past periods
can only be explained by some law of evolution, it seems at the
same time certain that there has always been some other deeper
and higher law at work, on the nature of which it would be futile
to speculate at present.
Not only do we find that the animals of each successive formation
become gradually more and more like those now existing upon the
globe, as we pass from the older rocks into the newer, but we also
find that there has been a gradual progression and development
in the _types_ of animal life which characterise the geological
ages. If we take the earliest-known and oldest examples of any
given group of animals, it can sometimes be shown that these
primitive forms, though in themselves highly organised, possessed
certain characters such as are now only seen in the _young_ of
their existing representatives. In technical language, the early
forms of life in some instances possess "_embryonic_" characters,
though this does not prevent them often attaining a size much
more gigantic than their nearest living relatives. Moreover, the
ancient forms of life are often what is called "comprehensive
types"--that is to say, they possess characters in combination
such as we nowadays only find separately developed in different,
groups of animals.
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