EVOLUTIONAL GEOLOGY. 305 



.short of eternity, is sufficient to account for the evolution of the living- 

 world. If the little tongue-shell, Lingula, has endured with next to 

 no perceptible change from the Cambrian down to the present day, 

 how long, it is sometimes inquired, would it require for the evolution 

 of the rest of the animal kingdom? The reply is simple. The cases 

 are dissimilar, and the same record which assures us of the persistency 

 of the Lingula tells us in language equally emphatic of the course of 

 evolution which has led from the lower organisms upward to man. 

 In recent and Pleistocene deposits the relics of man are plentiful. In 

 the latest Pliocene they have disappeared, and we encounter the 

 remarkable form P'lihceanthropus. As we descend into the Tertiary 

 systems the higher vuiammals are met with, always sinking lower and 

 lower in the scale of organization as they occur deeper in the series, 

 till in the Mesozoic deposits they have entirely disappeared, and their 

 place is taken by the lower mammals, a feeble folk, offering little 

 promise of the future the}^ were to inherit. Still lower, and even 

 these are gone; and in the Permian we encounter reptiles and the 

 ancestors of reptiles, probabl}^ ancestors of mammals, too; then into 

 the Carboniferous, where we find amphibians, but no true reptiles; 

 and next into the Devonian, where fish predominate, after making 

 their earliest appearance at the close of the Silurian times; thence 

 downward, and the vertebrata are no more found — we trace the evolu- 

 tion of the invertebrata alone. Thus the orderly procession of organic 

 forms follows in precisely the true phylogenetic sequence; inverte- 

 brata first, then vertebrates, at first fish, then amphibia, next reptiles, 

 soon after mammals, of the lowlier kinds first, of the higher later, and 

 these in increasing complexit}^ of structure till we finalh^ arrive at 

 man himself. While the living world was thus unfolding into new 

 and nobler forms, the immutable Z/;^(/?^/rf simply perpetuated its kind. 

 To select it or other species equally sluggish, as the sole measure of 

 the rate of biologic change would seem as strange a proceeding as to 

 confound the swiftness of a river with the stagnation of the pools that 

 lie beside its banks. It is occasionally olijected that the story we have 

 drawn from the paleontological record is mere myth or is founded 

 only on negative evidence. Cavils of this kind prove a double misap- 

 prehension, partly as to the facts, partly as to the value of negative 

 evidence, which may be as good in its way as any other kind of 

 evidence. 



Geologists are not unaware of the pitfalls which beset negative evi- 

 dence, and they do not conclude from the absence of fossils in the 

 rocks which underlie the Cambrian that pre-Cambrian periods were 

 devoid of life; on the contrary, they are fully persuaded that the seas 

 of those times were teeming with a rich variety of invertebrate forms. 

 How is it that, with the exception of some few species found in beds 



