10 THE LIVING WORLD. 



starting-point. It happens, however, that it is of 

 just these early periods that embryology gives us the 

 clearest account. For various practical reasons, 

 embryologists have largely confined themselves to 

 the study of the early stages of the development. 

 These stages teach of the relations of types to each 

 other, and of the separation of the types from a 

 common starting-point. In other words, they give 

 us an idea of just that part of the history of life that 

 we fail and shall forever fail to get from the study of 

 fossils. 



Closely allied to the study of embryology is that 

 of anatomy. It has been determined that the em- 

 bryological history of the higher animals of a class 

 passes through stages which are represented by the 

 adults of lower animals of the same class. This of 

 course renders the study of lower types a valuable 

 aid in tracing the history of the higher ones. It 

 is also recognized to-day that relations between 

 animals represent community of descent. When 

 we find two animals closely related anatomically we 

 interpret the fact as indicating a recent common 

 point of origin. Anatomical relationship thus rep- 

 resents blood relationship, and if we have the former 

 we can interpret the latter. It will, of course, follow 

 that the study of anatomical relations will teach us 

 history. 



The study of anatomy and embryology cannot be 

 isolated from each other. Together they give us an 

 idea of the relations of types to each other, and 

 enable us to draw up a picture of early development 

 of life. 



