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In any case, the changes which would confer advantage in the struggle for existence may take place in any direction—with, or against, or at right angles to the stream of progress. By means of those which march with that stream, the upper level of life’s attainment is raised. But the struggle still goes on: and again, starting from this new condition, there will be variations in every direction which will have survival value, and some of these will be progressive; and so the upper level will be once more raised.

The process will take time, for, whatever theory of variation we may hold[8]—the old idea of small continuous variations; or that of large mutations big enough to produce new species at one jump; or the most probable theory of numerous small mutations—they one and all must grant that the largest variation occurring at one time in a living species is infinitesimal in comparison with the secular changes of evolution.

There will further be a premium upon progressive changes, since a progressive change will generally land its possessor in virgin soil, so to speak; if not in an actually new physical environment, then in a biologically new situation. The placental mammal occupies the same dry land as did the wonderful reptilian types of the Secondary epoch. But constant temperature and embryonic nutrition within its mother provide delicately adjusted conditions in the early phases of development which in their turn enabled a more elaborate and more delicately responding brain machinery to be constructed in development, and so advanced their possessors on to new shores of control and independence.