84 Trans. Acad. Sci. of St. Louis. 



The discovery and interpretation of homologous structures, 

 now called homogenetic structures, gave to comparative anat- 

 omy a new goal, and led to a hitherto undreamed-of expansion 

 of the study. 



Before Darwin's work the embryologist Von Baer had 

 announced his discovery that the higher animals pass 

 through, in their individual development, stages which 

 correspond more or less closely with the adult grade of 

 organization of lower forms; but although the discoverer 

 of a great law, Von Baer had no notion of its impor- 

 tant meaning, nor had any of his contemporaries, except 

 that many regarded it merely as illustrating the general 

 harmony of plan in creation. Fritz Miiller, one of the most 

 ardent of Darwin's early supporters, from his observations 

 upon the development and life history of Crustacea, was the 

 first to point out its significance for the evolutionary theory. 

 It is now known as the Law of Recapitulation, or the Bio- 

 genetic Law as Haeckel called it, and in general it states the 

 now well-known fact that *' an animal in its growth from the 

 egg to the adult condition tends to pass through a series of 

 stages which are recapitulative of the stages through which 

 its ancestry has passed in the historical development of the 

 species from a primitive form; or more shortly, that the de- 

 velopment of the individual (ontogeny) is an epitome of the 

 development of the race (phylogeny)." As an animal in its 

 development is believed to retrace, as it were, its line of 

 descent, it can be readily seen what an impetus the formula- 

 tion of this principle gave to the study of embryology, for 

 there could be found the actual record, often obscured, modi- 

 fied and blurred it is true, but, nevertheless, a more or less 

 complete record of its ancestral history. For many years 

 afterwards the greatest attention was directed to the working 

 out of phylogenies or ancestral histories through the study of 

 comparative anatomy and embryology; and as a result of 

 these researches, carried on by zoologists over the entire world, 

 the growth of our knowledge in these subjects was stupen- 

 dous. As every iepoch-making discovery by a master-mind has 

 set the trend of investigation for a long period following, so 

 the study of phylogeny dominated zoology from Darwin's 



