GENERAL OBSERVATIONS. 1031 



cated so plainly by the very gradual steps in the successional lines ; the 

 progress of rudimentary organs may so often be traced from an early con- 

 dition of good size to that of rudiments, and variations in existing species 

 are so often wide and perplexing to the systematist, that the evidence in 

 favor of evolution by variation is now regarded as essentially complete. 



The argument from the facts presented on page 929, respecting the 

 descent of the Horse, is strengthened by the occurrence among modern 

 horses occasionally of a small pair of hoofs growing from the extremity of 

 the splint bones of each foot — the old toes lost by descent back again ; and 

 more rarely by the growth of a full-sized toe from one of these bones, on all 

 the feet, approximating thus to horses of the later Tertiary (Marsh, Am. 

 Jour. Sc, xliii. 1892). Birds, now standing apart from other Vertebrates so 

 stiffly, as animals with feathers, short tails, and bills without teeth, in former 

 times had teeth in their jaws, and long tails, like Eeptiles. Moreover, in 

 the Reptilian age, there were biped Eeptiles, with the hollow bones and some 

 other characteristics of Birds ; and also Mammals that laid eggs like Birds 

 and Reptiles, — as they continue now to do in Australia. 



There are, however, some large blanks in the series which are yet unex- 

 plained, although investigators have been at work over the subjects for scores 

 of years. One of these is the apparently sudden appearance of plants of the 

 tribe of Angiosperms, the most common kind of Recent time, in the Lower 

 Cretaceous ; another, the still more remarkably abrupt introduction of ordi- 

 nary or placental Mammals as successors to the Marsupials at the commence- 

 ment of the Tertiary ; another, the introduction of well-characterized Fishes, 

 without the discovery of their precursors. Such facts excite, at the present 

 time, interest in further study, but not doubts as to the general system of 

 progress. Already a small slender fossil, with a blade-like sculling tail and 

 terminal mouth, — the Palceospondylus Gunni, from the Devonian of Caith- 

 ness, Scotland, — has been described as probably a primeval Lamprey (an 

 eel-like Cyclostome, page 403). But, if correctly referred, there is still a 

 very wide interval between it and the early Placodernis. 



Some other general facts respecting successional lines, are the following : 



The lines of succession seldom connect the grander divisions of classes or tribes. 

 None lead directly from Macrural to Brachyural Crustaceans, or from Amphipod to Isopod 

 kinds. Instead, the group of Anomourans, intermediate between the two tribes first men- 

 tioned, was the course of successional lines in geological history, and of branches to both 

 the Macrurans and Brachyurans. In a similar way the Anisopods, intermediate between 

 the Isopods and Amphipods, or the typical Tetradecapods, were the source of branches to 

 tliese tribes. The principle is in accordance with tliat respecting comprehensive or syn- 

 thetic types, for the Anisopods and Anomourans are of this nature. A line leads direct 

 from the higher Ganoids to the Amphibians ; but, instead of lines from Amphibians to 

 Reptiles, and thence to Birds or to Mammals, all three groups — Reptiles, Birds, and 

 Mammals — were probably derived directly from the Amphibians. Instead of succes- 

 sional lines between Ungulates, Carnivores, and Quadrumana, these three groups were 

 probably derived, as Cope has remarked, from some common tribe in the earliest Eocene. 

 No successional lines among Insects appear to have passed between the higher tribes of 



