ROSS — ON EVOLUTION. 411 



ment until arriving at maturity ; and all that is known of the 

 animals that have existed on the Earth indicate that the metamor- 

 phoses, which each mature animal now living has undergone, (in 

 its individual development,) are types of the changes which have 

 taken place in the Kingdom, Sub-kingdom, Class, Sub-class, Order, 

 Sub-order, Family, Sub-family, Genus, Sub-genus, Species, Sub- 

 species, Variety, and Sub-variety, to which it may belong. 



The distinctive peculiarity of existing animals as compared with 

 those of past Epochs, therefore, is that their organization is more 

 specialized, so that as we go backward in time the distinctive 

 peculiarities of the natural groups gradually disappear, intermediate 

 forms and increasingly generalized or " synthetic " types continually 

 appear, bridging over apparent chasms. Thus the Genera Equus 

 and Elephas, each consisting of but a few^ existing Species and 

 widely separated from each other, and from every other Genus of 

 living animals, are found to be in close relation with many allied 

 and intermediate forms, the remains of which are found in the rocks 

 of the Kecent, the Quaternary, and the Tertiary Periods ; the types 

 becomino; more and more svnthetic as we go backward in time, and 

 the relative size of the brain cavity gradually diminishing, until in 

 the earliest Tertiary it becomes comparable to that of the Eeptilia. 

 The most remarkable diSerentiation in the Equine family is in the 

 structure of the foot ; passing gradually from the four toed Genus 

 Orohippiis of the Eocene, through such intermediate forms as the 

 three-toed genus AjichitJiermm of the Miocene, and Hipiparion of 

 the Pliocene, which had three toes, but only the middle one well 

 developed, the other two not reaching the ground, to its present 

 representative Equus, including the Horse, Zebra, &c., with its 

 single toed foot. 



Birds are a highly specialized Class of Vertehrata, having how- 

 ever closer structural affinities with the Chelonia than would be 

 supposed from external appearance. One of their marked peculiari- 

 ities is that they are all toothless. Few remains of the earlier Birds 

 have yet been found, but among them is Ichthyornis dispar, of the 

 Cretaceous Period, which shews a complete set of teeth ! The 

 embryo of certain living Birds also have teeth, thus illustrating the 

 law that the metamorphoses which the existing individual undergoes 



