INTRODUCTION. / 



is universally accepted in the biological world, and 

 we may assume with confidence that it will be a faith- 

 ful guide in the study of the history of life, at least 

 in so far as animals are concerned. 



Embryology, as a source of history, offers certain 

 advantages over the record of fossiliferous rocks, 

 and at the same time it is open to many difficulties 

 of its own. If the embryology of an animal repeats 

 the past history of its ancestors, and if all animals 

 have descended from various common points of 

 origin in the past, it would seem that we should 

 only have to study the embryology of a few animals 

 from each great type of the life in order to deter- 

 mine accurately the history of life in the past ages. 

 There are two facts, however, which render such a 

 simple proceeding impossible. 



First : The embryological history of an animal 

 lasts a few days or a few weeks ; the past history of 

 life has taken thousands of centuries. It is plain 

 that the embryological history of animals is too short, 

 and the past history of the life of the world is too 

 long to make it possible that the former should 

 retain a record of all the details of the latter. In 

 the epitome of the history which is presented to 

 us by the development of an animal, we find only 

 a few of the salient points in the history preserved, 

 which we may suppose represent such epochs of 

 the past as have been of the greatest importance, 

 and have therefore left the most lasting impres- 

 sion. In the embryological history we find that 

 details are left entirely unrecorded. The study of 

 an embryonic stage can give no idea of the actual 



