Page:Popular Science Monthly Volume 3.djvu/333

Rh every case of automatic Mental activity?—whether this be left altogether uncontrolled, or be in subjection to the will.

9. When a series of Physical sequences comes to be established by the Habitual action of the Cerebrum in particular modes directed or permitted by the Will, is it not consonant to all Physiological probability that the tendency to similar sequences should be hereditarily transmitted, like the tendency to bodily habits?—Contemporary Review.

 

N the vegetable world, limits of growth and life are strangely diversified. Multitudes of forms mature and perish in a few days or hours; while others, whose beginning was in a remote antiquity, have survived the habitual period of their kind, and still enjoy the luxuriance of their prime. Some species of unicellular plants are so minute that millions occur in the bulk of a cubic inch, and a flowering plant is described by Humboldt, which, when fully developed, is not more than three-tenths of an inch in height. On the other hand, we have the great Sequoia, whose mass is expressed by hundreds of tons, and specimens of the Eucalyptus, growing in the gulches of Australia, surpass in height the dome of St. Peter's.

Some of the Fungi mature between the setting and rising of the sun, while the oak at our door, which awakens the memories of our childhood, has not perceptibly changed in bulk in half a century. Trees grow more slowly as they increase in age. Nevertheless, it is certain that growth continues while they continue to live. The development of foliage implies interstitial activity and organization of new material. In its vital processes there is little expenditure of force or waste of substance. Its functions are essentially constructive, and its growth and age are apparently without limits, excepting such as arise from surrounding conditions. Thus many trees represent centuries, and have a permanence that is astonishing and sublime. Travellers stand awe-struck before the monuments which for forty centuries have kept watch by the Nile, but the oldest of these may not antedate the famous dragon-tree of Teneriffe. It is not surprising that the ancients considered trees "immortal," or, as "old as Time."

But, if the life of the tree is continuous, its leaves—the organs of its growth—have their periods of decay, and are types of mortality. The life of man is likened to the "leaf that perishes." In an animal, the vital processes are carried on by a single set of organs, the impairment of which limits the period of its life. With the tree, decay