578 THE POPULAR SCIENCE MONTHLY, 



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 mol- 

 lusks, 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, iulidae — 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 subordi- 

 nate 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 trans- 

 mission oi form and life has been actually efiected. It would be un- 

 reasonable 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 our- 

 selves Anglo-Saxons. Time necessarily devoured these when the pe- 

 riod of life was finished. So has it done with species; for, as the spe- 

 cies is continued through individuals, and always icith variatioii^ the 

 distance of removal from any special form is only a question of time. 

 The cosmos is always 



" Ein wechselnd Weben, 

 Ein gltihend 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 



