Evolution is, therefore, not a "mere description" of the procession of living things; it is to a great extent an explanation of the procession. When, however, we come to apply these general principles to certain aspects of the advance in organisation we find fundamental differences of opinion among biologists, which must be noted. As Sir E. Ray Lankester recently said, it is not at all true that Darwinism is questioned in zoology to-day. It is true only that Darwin was not omniscient or infallible, and some of his opinions are disputed. Let me introduce the subject with a particular instance of evolution, the flat-fish. This animal has been fitted to survive the terrible struggle in the seas by acquiring such a form that it can lie almost unseen upon the floor of the ocean. The eye on the under side of the body would thus be useless, but a glance at a sole or plaice in a fishmonger's shop will show that this eye has worked upward to the top of the head. Was the eye shifted by the effort and straining of the fish, inherited and increased slightly in each generation? Is the explanation rather that those fishes in each generation survived and bred which happened from birth to have a slight variation in that direction, though they did not inherit the effect of the parent's effort to strain the eye? Or ought we to regard this change of structure as brought about by a few abrupt and considerable variations on the part of the young? There you have the three great schools which divide modern evolutionists: Lamarckism, Weismannism, and Mendelism (or Mutationism). All are Darwinians. No one doubts that the flat-fish was evolved from an ordinary fish--the flat-fish is an ordinary fish in its youth--or that natural selection (enemies) killed off the old and transitional types and overlooked (and so favoured) the new. It will be seen that the language used in this volume is not the particular language of any one of these schools. This is partly because I wish to leave seriously controverted questions open, and partly from a feeling of compromise, which I may explain. [*] * Of recent years another compromise has been proposed between the Lamarckians and Weismannists. It would say that the efforts of the parent and their effect on the position of the eye--in our case--are not inherited, but might be of use in sheltering an embryonic variation in the direction of a displaced eye.