126 EVOLUTION OF TO-DAY. 



well-known parallelism between embryology and 

 paleontology. There is hardly a class in which 

 some such parallelism cannot be distinctly shown, 

 and many instances are very striking. It is seen 

 among the mollusks, particularly in the class Cephal- 

 opoda ; it is seen among the Crustacea, the Decapods 

 showing very beautifully a paleontological develop- 

 ment agreeing with their embryology. Vertebrates, 

 perhaps, give the best illustrations of all, as can be 

 seen by examining the various works on fossil ver- 

 tebrates, from the time of Louis Agassiz until now. 

 When, therefore, we take all of the facts together, see- 

 ing that with our imperfect knowledge of fossils we 

 find a general parallel everywhere expressed, we can 

 safely say that the investigations of the last twenty 

 years in paleontology have been slowly but per- 

 sistently strengthening the belief in the truth of the 

 hypothesis. While the evidence is scanty, it all 

 bears in one direction. 



Embryology Compared with a Hypothetical History. 



Fossils are too scanty, and paleontological evi- 

 dence has been hitherto too imperfect, for a demon- 

 stration of the truth of the hypothesis. It is 

 necessary, therefore, to search for proof in some 

 other direction. One method of doing this is to 

 compare embryology with the hypothetical history 

 which the study of adult forms with their relations 

 to each other would lead us to construct. If the 

 two agree, we may rest assured of the truth of the 

 hypothesis; if they disagree, something must be 

 wrong. Two species of the same genus must, ac- 



