818 THE POPULAR SCIENCE MONTHLY. 



and paleontology renders such explanations almost superfluous. 

 Geology, aided by the deep-sea explorations, has come to a better 

 comprehension of the mechanism of sediments, and it knows what 

 it may expect to find in the rocky archives of the earth, and what 

 it may not ; and, on the other side, the discovery of the missing 

 links between past and present has been going on of late with 

 such a rapidity as has outstripped the most sanguine expectations. 

 Our museums already contain whole series of fossil organisms 

 which almost step by step illustrate the slow evolution of large 

 divisions of both animals and plants ; our present mammals al- 

 ready have been connected by intermediary forms with many of 

 their Tertiary ancestors ; and the paleontologist can already trace 

 the pedigree of birds, and even mammals, as far back as the liz- 

 ards of the Secondary period not merely deducing it from em- 

 bryological data, but by showing the real beings which once 

 breathed and moved about upon earth. 



At the same time one point of great moment for the theory of 

 evolution, and only alluded to by Darwin, has been brought into 

 prominence. The part played by migrations in the appearance of 

 new species has been rendered quite obvious. Thus we know per- 

 fectly well that the ancestors of our horse migrated over both 

 Americas, Asia, Europe, Africa, and probably back to Asia, and 

 that each step in those migrations was marked, by the apparition 

 of some new characters which are now distinctive of the horse. 

 The same remark applies to the mastodons and their descendants, 

 the elephants ; to the common ancestors of the camel and the 

 llama, and to the Ungulata altogether. It may be taken now as a 

 general rule that the evolution of new species chiefly took place 

 when the old ones were compelled to migrate to new abodes, and 

 to stay there for a time in new conditions of climate and general 

 surroundings. The intermediate forms have not been extermi- 

 nated on the spot ; and if we want to obtain the intermediate links 

 between two allied species, the relics of which are found in two 

 geological formations of a given country, we must ransack for 

 fossils all the five continents upon which the intermediate links 

 have been scattered. This is why the discovery of intermediate 

 types has gone on so rapidly since North America, South Africa, 

 South America, New Zealand, and partly Asia began to be thor- 

 oughly explored by experienced paleontologists. 



Many of the " missing links " were discovered, as is known, 

 in Darwin's lifetime. Thus, the first really bird-like, feathered 

 lizard, the ArchcBopterix, was unearthed as early as 1862 ; and 

 eight years later, Prof. O. C. Marsh already described, from the 

 Upper Cretaceous beds of North America, two more lizard-birds, 

 one of which (Hesperornis) must have resembled our present fish- 

 eating divers, while the other (Ichthyomis), provided with power- 



