300 EVOLUTION AND ANIMAL LIFE 



garded as indicative of the actual ancestry, from which the 

 later types have diverged. 



The persistence of heredity is the basis of the parallelism 

 between geological and embryonic series. By its influence 

 ancestral traits are repeated in the embryo, even though the 

 characters thus produced give way in later development to 

 further specialization or growth along other lines. This great 

 truth has been stated in these words: "The life history of the 

 individual is an epitome of the life history of the group to 

 which it belongs." This statement is only true when stated 

 very broadly, for there are many exceptions or modifications. 

 The embryonic or larval animal is subject to almost endless 

 secondary changes and adaptations whenever these changes 

 are for the advantage of the animal. In general, the simpler 

 the structure of the animal and the less varied its relations in 

 life, the more perfectly are these ancient phases of heredity 

 preserved in the process of development. In such case, the 

 more perfect is the parallelism between the development of 

 the individual and the succession of forms in geologic time. 



It is not always true that the recent representatives of a 

 group are higher in a morphological sense than some or all of 

 the earlier members. They are, however, in all cases farther 

 from the original or parent stock. In many groups there is a 

 progress, seemingly rapid, toward a high degree of specializa- 

 tion followed by the disappearance of the highly organized 

 types, while forms of low development sometimes even those 

 of primitive character may remain in abundance. The evolu- 

 tion of the group of Brachiopods is an illustration of this. The 

 group is represented in the Lower Silurian by numerous genera 

 of simple structure, as Lingula, Terebratula and the like. It 

 culminates in the Carboniferous age with complex genera as 

 Spirifer, Products is, Orthis, while the modern representatives 

 Lingulella, Terebratulina, Waldheimia,, etc., are little more ad- 

 vanced than the primitive forms. Similar phases have charac- 

 terized the appearance, culmination, and relative extinction 

 of the trilobites, the crinoids, the ammonites, and other groups. 

 The total extinction of any large group has not usually taken 

 place. Usually a few species have remained, thus giving us a 

 better clew to the life history and development of the group 

 than we should otherwise possess. 



One feature shown in many groups of extinct animals has 



