INTRODUCTION 13 



evidence of gradual modification in structure ; thus the 

 earliest known forerunner of the horse, found in the 

 Eocene beds, possessed five toes, and was succeeded in 

 later times by forms with successively fewer toes, until in 

 the Pliocene, the existing type (Equus), with only one toe 

 and splint-bones, appeared ; other gradual changes also 

 occurred in the character of the teeth, etc. 



In the development and growth of an animal, various 

 stages, which often present some resemblance to the 

 adults of other animals, are passed through. In some 

 cases these stages in the life of the individual are also 

 similar to those which occurred in the history of its race, 

 as shown by the geological record. This agreement gives 

 some support to the ' recapitulation theory,' which sup- 

 poses that the changes passed through in the development 

 of the individual (ontogeny) are, in a general way, a rapid 

 and incomplete repetition of those which occurred in its 

 race-history (phylogeny). 



In a natural classification of animals an attempt is 

 made to place together in the same group those forms 

 which are connected by descent ; such a classification, if 

 perfect, would be of the nature of a genealogical tree. 

 Each main division is termed a Phylum, and is divided and 

 subdivided into smaller and smaller groups, known as 

 Classes, Orders, Families, Genera, and Species. A species 

 includes a group of individuals which appear to have 

 descended from the same ancestors and can give rise to 

 offspring which are fertile among themselves ; such indi- 

 viduals usually differ from one another to only about the 

 same degree that offspring of the same parents may differ. 

 One species is generally distinguished from another by 

 such characters as ornamentation, shape, relative propor- 



