III
NATURAL SELECTION AND THE CHANGES OF SPECIES
NOW here it will be well to put plainly certain general facts about this new thing, life, that was creeping in the shallow waters and intertidal muds of the early Palæozoic period, and which is perhaps confined to our planet alone in all the immensity of space.
Life differs from all things whatever that are without life in certain general aspects. There are the most wonderful differences among living things to-day, but all living things past and present agree in possessing a certain power of growth, all living things take nourishment, all living things move about as they feed and grow, though the movement may be no more than the spread of roots through the soil, or of branches in the air. Moreover, living things reproduce; they give rise to other living things, either by growing and then dividing or by means of seeds or spores or eggs or other ways of producing young. Reproduction is a characteristic of life.
No living thing goes on living forever. There seems to be a limit of growth for every kind of living thing. Among very small and simple living things, such as that microscopic blob of living matter the Amœba, an individual may grow and then divide completely into two new individuals, which again may divide in their turn. Many other microscopic creatures live actively for a time, grow, and then become quiet and inactive, enclose themselves in an outer covering and break up wholly into a number of still smaller things, spores, which are released and scattered and again grow into the likeness of their parent. Among more complex creatures the reproduction is not usually such simple division, though division does occur even in the case of many creatures
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