Page:Popular Science Monthly Volume 44.djvu/28

20. Nay, we have but to cast our eyes over the rest of the world and cyclical change presents itself on all sides. It meets us in the water that flows to the sea and returns to the springs; in the heavenly bodies that wax and wane, go and return to their places; in the inexorable sequence of the ages of man's life; in that successive rise, apogee, and fall of dynasties and of states which is the most prominent topic of civil history.

As no man, fording a swift stream, can dip his foot twice into the same water, so no man can with exactness affirm of anything in the sensible world that it is. As he utters the words, nay, as he thinks them, the predicate ceases to be applicable; the present

germ (A) gives rise to tissues and organs; while another part (B) remains in its primitive condition, or is but slightly modified. The moiety A becomes the body of the adult and, sooner or later, perishes, while portions of the moiety B are detached and, as offspring, continue the life of the species. Thus, if we trace back an organism along the direct line of descent from its remotest ancestor, B, as a whole, has never suffered death; portions of it only have been cast off and died in each individual offspring.

Everybody is familiar with the way in which the "suckers" of a strawberry plant behave. A thin cylinder of living tissue keeps on growing at its free end, until it attains a considerable length. At successive intervals it develops buds, which grow into strawberry plants: and these become independent by the death of the parts of the sucker which connect them. The rest of the sucker, however, may go on living and growing indefinitely, and, circumstances remaining favorable, there is no obvious reason why it should ever die. The living substance B in a manner answers to the sucker. If we could restore the continuity which was once possessed by the portions of B, contained in all the individuals of a direct line of descent, they would form a sucker, or stolon, on which these individuals would be strung, and which would never have wholly died.

A species remains unchanged so long as the potentiality of development resident in B remains unaltered; so long, e. g., as the buds of the strawberry sucker tend to become typical strawberry plants. In the case of the progressive evolution of a species, the developmental potentiality of B becomes of a higher and higher order. In retrogressive evolution the contrary would be the case. The phenomena of atavism seem to show that retrogressive evolution—that is, the return of a species to one or other of its earlier forms—is a possibility to be reckoned with. The simplification of structure which is so common in the parasitic members of a group, however, does not properly come under this head. The wormlike, limbless Lernæa has no resemblance to any of the stages of development of the many-limbed active animals of the group to which it belongs.