The mollusks, clams, and snails took an easier, down-hill road.  They formed a shell, and it developed large enough to cover them.  It hampered and almost destroyed locomotion and reduced nerve to a minimum.  But nerves are nothing but a nuisance anyhow.  And why should they move?  Food was plenty down in the mud, and if danger threatened, they withdrew into the shell.  They stayed down in the mud and let the world go its way.  If grievously afflicted by a parasite they produced a pearl—­to save themselves from further discomfort.  They developed just enough muscle and nervous system to close the shell or drag it a little way; that was all.  Digestion and reproduction retained the supremacy.  They were fruitful and multiplied, and produced hosts of other clams and snails.  The present was enough for them and they had that.

For if the winner in the struggle for existence is the one who gains the most food, the most entire protection against discomfort, danger from enemies or unfavorable surroundings, and the most fruitful and rapid reproduction—­and these are all good—­then the clam is the highest product of evolution.  It never has been surpassed—­I venture to say it never can be—­except possibly by the tape-worms.  I can never help thinking with what contempt these primitive oysters, if they had had brains enough, would have looked down upon the toiling, struggling, discontented, fighting, aspiring primitive vertebrates.  How they would have wondered why God allowed such disagreeable, disturbing, unconventional creatures to exist, and thanked him that he had made the world for them, and heaven too, if there be such a place for mollusks.  Their road led to the Slough of Contentment.

But even in molluscan history there was a tragic chapter.  The squids and cuttle-fishes regained the swimming life, and in their latest forms gave up the protective shell.  But its former presence had so modified their structure that any great advance was impossible.  It was too late.  The sins of the fathers were visited upon the children in the thousandth generation.

The vertebrate developed an internal skeleton.  This was necessarily a slow growth, and the type came late to supremacy.  The longitudinal muscles are arranged in heavy bands on each side of the back, and the animal swims rapidly.  The sense-organs are keen.  The brain contains the ganglia of several or many segments and is highly differentiated.  It has a special centre of perception, thought, and will; it is an organ of mind.  The vertebrate has the physical and mental advantages of large size.