, and consequently resemble,
the common progenitor of groups, since become widely divergent. Extinct
forms are seldom directly intermediate between existing forms; but are
intermediate only by a long and circuitous course through other extinct
and different forms. We can clearly see why the organic remains of
closely consecutive formations are closely allied; for they are closely
linked together by generation. We can clearly see why the remains of an
intermediate formation are intermediate in character.

The inhabitants of the world at each successive period in its history
have beaten their predecessors in the race for life, and are, in so
far, higher in the scale, and their structure has generally become more
specialised; and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession
of the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.

If, then, the geological record be as imperfect as many believe, and
it may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection are
greatly diminished or disappear. On the other hand, all the chief laws
of palaeontology plainly proclaim, as it seems to me, that species have
been produced by ordinary generation: old forms having been supplanted
by new and improved forms of life, the products of variation and the
survival of the fittest.

CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences in physical
conditions--Importance of barriers--Affinity of the productions of the
same continent--Centres of creation--Means of dispersal by changes of
climate and of the level of the land, and by occasional means--Dispersal
during the Glacial period--Alternate Glacial periods in the North and
South.

In considering the distribution of organic beings over the face of
the globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be wholly accounted for by climatal and other physical cond