INTRODUCTION. 9 



have the satisfaction of feeling that we are dealing 

 with a type which was of importance in the history 

 of animals, and therefore of more far-reaching sig- 

 nificance than any fossil or score of fossils that we 

 could pick out of the rocks. The relations expressed 

 by corresponding stages of different animals are very 

 suggestive, and a short study of embryology will 

 tell more of the true history of animals than the 

 collection of thousands of fossils. 



Second : The second fact that renders the study of 

 history from this source a difficult matter is the very 

 common falsification of the true record. It is un- 

 doubtedly the law that animals tend to repeat past 

 history in their embryology, but it is also true that not 

 infrequently various modifying circumstances occur 

 to prevent the history being correctly recorded. 

 The true record of past history is thus commonly 

 obscured by numerous departures caused by the 

 conditions surrounding the embryo, and this of 

 course renders the reading of the history a difficult 

 matter. The embryologist has, however, methods 

 of correcting these errors in large measure, and of 

 reaching trustworthy conclusions. 



In spite of these two objections, embryology is of 

 the very greatest assistance in enabling us to read 

 the past history of animals, and to a less extent that 

 of plants. Especially is this true of the early stages 

 of the history. As we have above seen, the fossil 

 records of animals become less and less satisfactory 

 as we go farther back in history, and they finally 

 cease altogether, long before we come to anything 

 like a union of the converging types into a common 



