THE ANIMAL KINGDOM 1297 



nores development; and it was reserved for Von 

 Baer to show the importance of developmental 

 studies, and to inaugurate the marvelous series of 

 researches which, in the course of the last fifty years, 

 have made us acquainted with the manner of de- 

 velopment of every important group of animals. 

 The splendid researches of Cuvier gave birth to 

 scientific palaeontology, and demonstrated that, in 

 some cases, at any rate, extinct forms of life present 

 characters intermediate between those of groups 

 which are at present widely different. The investi- 

 gations of Agassiz upon fossil fishes tended in the 

 same direction, and further showed that, in some 

 cases, the older forms preserve, as permanent fea- 

 tures, structural characters which are embryonic and 

 transitory in their living congeners. Moreover, 

 Darwin, Owen, and Wallace proved that, in any 

 great area of geographical distribution, the later 

 tertiary extinct forms are clearly related to those 

 which now exist in the area. As Taxonomic investi- 

 gations increased in accuracy and in extent, the care- 

 ful examination of large suites of specimens revealed 

 an unexpected amount of variability in species ; and 

 Darwin's investigation of the phenomena presented 

 by animals under domestication proved that forms, 

 morphologically as distinct as admitted natural 

 genera, could be produced by selective breeding 

 from a common stock. 



The only genus of animals of which we possess a 

 satisfactory, though still not quite complete, ances- 

 tral history, is the genus Equus, the development of 

 which in the course of the Tertiary epoch from an 



