big enough to be visible to the unassisted eye. But the rule with almost all larger beings is that the individual grows up to a certain limit of size. Then, before it becomes unwieldy, its growth declines and stops. As it reaches its full size it matures, it begins to produce young, which are either born alive or hatched from eggs. But all of its body does not produce young. Only a special part does that. After the individual has lived and produced offspring for some time, it ages and dies. It does so by a sort of necessity. There is a practical limit to its life as well as to its growth. These things are as true of plants as they are of animals. And they are not true of things that do not live. Non-living things, such as crystals, grow, but they have no set limits of growth or size, they do not move of their own accord and there is no stir within them. Crystals once formed may last unchanged for millions of years. There is no reproduction for any non-living thing.
This growth and dying and reproduction of living things leads to some very wonderful consequences. The young which a living thing produces are either directly, or after some intermediate stages and changes (such as the changes of a caterpillar and butterfly), like the parent living thing. But they are never exactly like it or like each other. There is always a slight difference, which we speak of as individuality. A thousand butterflies this year may produce two or three thousand next year; these latter will look to us almost exactly like their predecessors, but each one will have just that slight difference. It is hard for us to see individuality in butterflies because we do not observe them very closely, but it is easy for us to see it in men. All the men and women in the world now are descended from the men and women of a.d. 1800, but not one of us now is exactly the same as one of that vanished generation. And what is true of men and butterflies is true of every sort of living thing, of plants as of animals. Every species changes all its individualities in each generation. That is as true of all the minute creatures that swarmed and reproduced and died in the Archæozoic and Proterozoic seas, as it is of men to-day.
Every species of living things is continually dying and being born again, as a multitude of fresh individuals.