Thus the strong plant with the abundant nectar would be cross-fertilized and would keep its pistil unharmed. It would be very likely to develop good strong seeds that would grow and again bear strong flowers with plenty of nectar. Now, remember the essential organs—that is, stamens and pistil—seem to find it a little easier to change than other parts of the plant; so it would not be surprising if in time some of the stamens were to become different. You see, the insects in visiting the flowers would irritate them more or less walking over them and clinging to them, and they would be likely to undergo change for this reason; and if it happened that in some flower a row of stamens got too full of sap to know what to do with themselves and so spread out a little broader and more leaf-like and kept their yellow stamen color or bleached-out white, that flower would be seen far and near and the insects would go straight to it, for insects have the sharpest kind of eyes for seeing bright colors a long way off. You see what would happen; all the flowers whose stamens had done so would be abundantly cross-fertilized,—that is, all their seeds would get fresh pollen from another strong plant, and the plants growing from these seeds would inherit the tendency of their parents to form petal-like parts from some of the stamens. The flower could well afford to lose part of its stamens for this purpose. Of course as time went on, these stamens, which were half petals, might develop more and more in the direction of signals,—that is, might become more and more perfect petals, finally losing all trace of their old life as stamens. Of course no one can say that is just the way it came about, but it is likely that in some such way it happened, for there are proofs of it which you may like to read when you grow older. So, you see, flowers _are_ nothing but leaves after all,—very much changed leaves, to be sure, but yet just leaves. Sometimes when plants and animals have changed into a new form, they change back again. We know some plants which once had petals but which have again lost their petals and gone back to a form which has no petals. Such backward changes we call retrogression, and it is sometimes difficult to find out whether a flower with no petals is a primitive form which for some reason has not changed or whether it is one which has changed and gone back again. Usually, though, we can find traces of petals and sepals in flowers which have retrogressed.