It
seems clear, at least, that two main branches, the Monotremes and
Marsupials, arose from the primitive mammalian root. Whether
either of these became in turn the parent of the higher mammals
we will inquire later. We must first consider the fresh series of
terrestrial disturbances which, like some gigantic sieve, weeded
out the grosser types of organisms, and cleared the earth for a
rapid and remarkable expansion of these primitive birds and
mammals.
We have attended only to a few prominent characters in tracing
the line of evolution, but it will be understood that an advance
in many organs of the body is implied in these changes. In the
lower mammals the diaphragm, or complete partition between the
organs of the breast and those of the abdomen, is developed. It
is not a sudden and mysterious growth, and its development in the
embryo to-day corresponds to the suggestion of its development
which the zoologist gathers from the animal series. The ear also
is now fully developed. How far the fish has a sense of hearing
is not yet fully determined, but the amphibian certainly has an
organ for the perception of waves of sound. Parts of the
discarded gill-arches are gradually transformed into the three
bones of the mammal's internal ear; just as other parts are
converted into mouth cartilages, and as--it is believed--one of
the gill clefts is converted into the Eustachian tube.
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