When the ancestral type is in some way inferior to the modern one, variation which consists in reverting to the former is often referred to as Degeneracy. There is reason to believe that discomfort and hardship of existence tend to produce variation of this kind--a fact of supreme importance, when the problem of Degeneracy is considered in connection with human life. When creatures begin to degenerate, it is, in fact, as if the species were saying to itself, "I have gone astray; let me retrace my steps along the road by which I came, and maybe I shall find comfort and safety; step by step I will try to go back to my ancestral form." Very rapid variation of any sort is indeed often a sign that the struggle for existence is too hard for the type in question. The palæontologist can tell us of types that present numerous variations before becoming extinct; while others, comfortably holding their own in the struggle for existence, remain practically unchanged during age after age of the geological record, and survive even up to the present day. We may borrow from commercial life a homely illustration that will explain this aspect of variation. When competition in trade is keen, the seller must have novelties; he will try all sorts, and find some good, some bad, some indifferent. If he now revives an out-of-date pattern of goods, for the sole sake of change, this is Degeneracy. But where, on the contrary, competition is dull, the same firm will turn out the same goods for a long period of time. There is an optimum in trade competition: a reasonable competition results in the production of sensible novelties, and consequent progress; but competition over-keen results in the production of rubbish, leading to eventual failure. So in the world of animal life; a certain degree of struggle for existence results in variation, establishment of new varieties, progress. A greater degree results in too rapid variation, new varieties that speedily perish, and finally, the extinction of the type. We have spoken of "varieties." Each of the domestic animals presents varieties, which are the cumulative result of the breeder's artificial selection of natural variations. Thus the Pug and the Collie for instance, are varieties of the Dog; the Bantam and the Dorking of the Fowl. Among wild animals, varieties are similarly produced by _natural_ selection, resulting from the "survival of the fittest." By degrees, intermediate forms are lost; and new species are established by the greater and greater divergence of varieties originally derived from one ancestral type. TABLE SHOWING THE POSITION IN CLASSIFICATION OF THE ANIMALS NAMED IN THE FOREGOING CHAPTER =Phylum= =MOLLUSCA=, or Shell-fish. =Class= GASTEROPODA, or Snail-like Shell-fish. =Sub-Class= ANISOPLEURA, or Unequal-sided Gasteropods. =Branch= STREPTONEURA, or Unequal-sided Gasteropods with nerves twisted into the shape of a figure of 8.