. ORIGIN AND EVOLUTION OF LIFE 315
aridity, of temperature, of chemical soil content; than in organisms like animals which secure their food compounds ready-made by the plants and possessing comparatively similar and stable chemical for- mtdae. Thus a plant transferred from one environment to another exhibits much more sudden and profound changes than an animal, for the reason that all the sources of plant energy are profoundly changed while the sources of animal energy are only slightly changed. The highly varied chemical sources of plant energy are, in other words, in striking contrast with the comparatively uniform sources of animal energy which are primarily the carbohydrates and the proteins formed by the plants.
In respect to character-origin, therefore, plants may in accord- ance with the de Vries mutation hypothesis exhibit discontinuity or sudden changes of form and function more frequently than animals. In respect to character-coordination, or the harmonious relations of all their parts, plants are inferior to animals only in their sole de- pendence on catalytic enzymes ; while animal characters are coordinated both through catalytic enzymes and through the nervous system. In respect to character-velocity, or the relative rates of movement of dif- ferent parts of plants both in individual development and in evolution, plants appear to compare very closely with animals.
This law of changes in character-velocity, both in individual de- velopment (ontogeny) and in racial development (phylogeny), is one of the most mysterious and diflScult to understand in the whole order of biologic evolution, for it is distinctively a chromatin phenomenon, although visible in protoplasmic form. Among plants it is illustrated by the recent observations of Coulter on the relative time of appear- ance of the reproductive cell organs (archegonia) in the two great groups of gymnosperms, the Cycads and the Conifers, as follows: in the Cycads, which are confined to warmer climates, the belated appear- ance of the archegonium persists; in the Conifers, in adaptation to colder climates and the shortened reproductive season, the appearance of the archegonium is thrust forward into the early embryonic stages. Finally, in the flowering plants (Angiosperms) the backward move- ment of this character continues until the third cellular stage of the embryo is reached. This is but one illustration among hundreds which might be chosen to show how character-velocity in plants follows ex- actly the same laws as in animals, namely, characters are accelerated or retarded in race evolution and in individual development in adap- tation to the environmental and individual needs of the organism. We shall see this mysterious law beautifully illustrated among the verte- brates, where of two characters, lying side by side, one exhibits inertia, the other momentum.
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