4. It is not impossible--indeed, it is in exact accordance with the laws of evolution--that organic forms are more permanent now than ever before. Evolution is a growth; the forces of growth must exhaust themselves. Evolution proceeds by constant differentiation and specialization, but extreme specialization always arrests evolution. In ontogenic evolution, for example, cell-structure becomes more and more specialized, but also thereby more and more rigid, and, when specialization is complete, evolution stops, and cell-forms are permanent. It is this which limits the cycle of every evolution. So is it precisely with evolution of the organic kingdom, except that the cycle is much longer. Here, also, every step is by specialization, and yet specialization fixes the form, and finally arrests the advance on that line. Thus, throughout the whole geological history of the earth, the larger number of forms, by specialization, become rigid and perish, while the fewer, more generalized, and more plastic forms take up the march and carry it forward a step, only to be themselves specialized and fixed. If we compare, again, to a tree: each twig finishes its growth, flowers, fruits, and dies; other buds take up the growth and carry it forward. By specialization the highest condition of a certain form of life is attained, but other possibilities are shut off. Extreme specialization is the flowering and fruiting--the end and completion of twig-life. Now, obviously, this specialization and respecialization can not go on forever. When it is complete in every direction it must cease, and forms become permanent, or else perish. When it flowers it must die.