8i8 THE POPULAR SCIENCE MONTHLY.
fication. It may be only one individual of many which, becomes modified and transmits the modification to descendants ; it may be but one species of a genus which, for similar reasons, supersedes the rest which become extinct in time proportioned to pro- lificacy.
There is no reason to suppose that the history of organic life has differed in this respect from that of inorganic. We need not discuss here the question of catastrophism and uniformitarianism in geology However much the latter prevails at the present time, both have doubtless operated in the past. Catastrophism would necessarily produce gaps, or saltations, in the palseontological record, as only the more plastic species would adapt themselves and survive under its influence. It is not gaps due to such causes that are here to be considered, however, but those which occur in uniform strata. Haldeman has most suggestively remarked that the same mineral will crystallize with three, six, or twelve angles, but not with five or seven, and he asks. Are the facts of organic morphism subject to less definite laws ? Cope has drawn another illustration from inorganic forces, in the three great changes in water, from solid, liquid, and vapor, which take place suddenly at what may be called two expression-points of the thermometer, the many intervening degrees involving no change, Ehythm or wave movement would seem to be a universal attribute of matter, whether organic or inorganic. The forces of nature are constant, but the phenomena induced are often paroxysmal. The progress- ive forces accumulate, while the conservative forces resist, until at last resistance gives way with comparative suddenness. There is every reason to believe that the life-movement, in its ascending complexity, has shared this common law. Accumulation is pro- portioned to the change in environment, and resistance to the age or rigidity of the organism. The latter may be strong enough to end in death or extinction, or it may break down, and, with com- paratively sudden yielding and conformity to necessity, burst the confines and begin a new series of variations and adaptations. In either case we have breaks, because the dying or dropping out of one type makes room for another more accommodating. Rapid evolution, from causes already discussed, implies gaps which must be marked according as the strength of the conservative forces and the violence of the final accommodation are great, and "because certain breaks are more apt to occur after long periods of stability. The break may be induced by changes in physical environment or without such change ; if the latter, it will more likely occur in some individual born with a marked departure from the type that gives it some advantage, and whose issue will in time supplant all other individuals. In either case we shall have, palseontologically, dis- tinct species or genera, one superposed on the other, without links.