In fact, it requires but little investigation to show that the one thing we can say with certainty about any living thing is that it cannot keep from changing.

Without change there can be no life.

Even the mineral cannot resist change, and the higher we go in the scale of being, the more varied, complex, and wonderful do the changes become.

Moreover, in progress and development among creatures of all grades we find two kinds of change--one slow, gradual, often almost imperceptible; and the other rapid, sudden and dramatic.

The latter occur at what are called "critical stages" of development.