of the blood. Comparative physiology, guided by the light of this metamorphosis of the segments, can have no difficulty in tracing the history of the heart back from mammals, and reptiles, and typical mollusks, to crustacea; and finding it at last reduced to its elementary form in the twofold pulsatile vesicle of the dorsal artery of each double segment of, for instance, iulidæ—lowest of the annulose series. The folding together and stricter union of these segments, as we have seen, bringing these pulsatile vesicles into juxtaposition, perfectly account for the two double hearts of bivalve mollusca, and of fishes; while the approximation and union on the median line of this double machinery of the mollusk explain the fourfold heart and the circulation of the highest types of life.
If you have followed the chain of evolution here briefly sketched, although in mere suggestions, you can have no difficulty in perceiving the unbroken succession of allied links, all the way from the lowest cell-formations up to man. Many forms, doubtless, belong to subordinate systems, and take no part, directly, in the chain; yet the study of all will enable us more fully to comprehend the whole process. In fact, we have no right to expect to find, unless perhaps in fossil forms, the precise species—the very links of the chain—by which the transmission of form and life has been actually effected. It would be unreasonable to require this of the advocates of evolution, since the exact contrary would follow, as an a priori conclusion, from the very terms of the proposition. It would be a wonder, indeed, in view of all the transitions and transmutations of matter, of force, of time, of place, of forms, if we could find, now living, a single species actually concerned in the long process of evolution—a single "bark which brought us hither." It is just as unreasonable as to demand of us to produce, alive, every individual through whom our descent has been accomplished—just as unreasonable as to demand the resurrection of all the members of our race, for the last thousand years, to prove ourselves Anglo-Saxons. Time necessarily devoured these when the period of life was finished. So has it done with species; for, as the species is continued through individuals, and always with variation, the distance of removal from any special form is only a question of time. The cosmos is always
"Ein wechselnd Wehen,
Ein glühend Leben,"
and, therefore, it takes but a few generations—a few thousands, or a few millions of years—to leave behind any specific type, as completely as the forgotten bones of our progenitors that lie hid in Batavian bogs. Why, with all the lights of human history, we cannot trace the line of any human family more than a few hundred years! And what is the historic period, upon which all doubts and objections are based, compared to the ages multitudinous that have passed away without recognition in human calendars? Take the simplest tree, a few years old. We see a crowning bud at the apex of its principal axis of
