EMBRYOLOGY IN CLASSIFYING ANIMALS. 139 



related agree in their development ; that species 

 which form a circumscribed group usually have a 

 circumscribed type of development. The embry- 

 ologist can predict with almost certainty exactly 

 what the development of any animal will be, if he 

 knows its adult relationship. So close is this par- 

 alleled, that scientists have considered themselves 

 justified in reversing the argument and in basing 

 their classifications upon embryology. Almost all 

 of the advances made in the classification of the 

 larger groups are made through the study of embry- 

 ology ; no classification is considered complete with- 

 out it; and in many points, indeed, the conclusions 

 as to relationship are based on this study alone. 

 And this fact is conclusive evidence that continued 

 investigations show, in spite of the numerous con- 

 tradictions, a parallelism between embryology and 

 that hypothetical history which we should build 

 from the study of adults. Von Baer certainly dis- 

 covered an important principle, although it no 

 longer stands as he formulated it. To-day it reads 

 something as follows: Closely related animals agree 

 in their development, while those distantly related 

 show no likeness except in the early stages ; but 

 since all of the members of the large groups of ani- 

 mals have had in the past a common history, it will 

 follow that they will have a common embryology, if 

 one is the repetition of the other. We thus explain 

 the fact that the so-called types of animals have 

 their own types of development. 



We may, then, safely assert that the investiga- 

 tions have strengthened almost to proof the funda- 



