But granting all that can be claimed for this sequence, have not the lower forms whose anatomy we have sketched—­worm, fish, and bird—­halted at various points along this line of march?  Yet they have evidently survived.  And if they have found safe resting-places, cannot higher forms turn back and join them?  In other words, is not degeneration easier than advance and just as safe?  What is the result if an animal tries to return to a lower plane of life or refuses to take the next upward step?  Generally extermination.  The very classification of worms in a number of small isolated groups, which must once have been connected by a host of intermediate forms, is indisputable proof of most terrible extermination.  They did not go forward, and the survivors are but an infinitesimal fraction of those which perished.  Let us take an illustration where palaeontology can help us.  The earth was at one time covered with marsupial mammals.  Some advanced into placental forms.  The great mass remained behind.  And outside of Australia the opossums are the only survivors of them all.  And this is only one example where a thousand could be given.  Place is not long reserved for mere cumberers of the ground.  There are so few exceptions to this statement that we might almost call it a law of biology.

Let us see how it fares with an animal which retreats to a lower plane of life.  A worm, rather than seek its own food, becomes a parasite.  It degenerates, but still is easily recognized as a worm.  A crustacean tries the same experiment, though living outside of its host instead of in it.  It sinks to a place even lower, if possible, than that of the parasitic worm.  A locomotive form becomes sessile.  It loses most of its muscles and the larger part of its nervous system; and even the digestive system, which it has made the goal of its existence, is inferior to that of its locomotive ancestors and relatives.  But to the vertebrate these lowest depths of stagnation and degeneration are, as a rule, impossible.  From true fish upward parasitism and sessile life are practically impossible.  Here stagnation and degeneration mean, as a rule, extinction.  Of all the relatives of vertebrates back to worms only the very aberrant lines of amphioxus and of the tunicata remain.  Of the rest not a single survivor has yet been discovered.  And yet what hosts of species must have peopled the sea.  The primitive round-mouthed fishes have practically disappeared.  The ganoids survive in a few species out of thousands.  The amphibia of the carboniferous and the next period and the reptiles of the mesozoic have disappeared; only a few feeble degenerate remnants persist.  And this was necessarily so.  Each advancing form crowded hardest on those which occupied the same place and sought the same food, that is, the members of the same species.  And the first to suffer from its competition were its own brethren.  Death, rarely commuted into life imprisonment, is the verdict pronounced on all forms which will not advance.  And does not the same law of advance or extinction apply to man?  What is the record of successive civilizations but its verification?