10 The American Naturalist. [January, 



Jackson and others, written in the last thirty years in this 

 country and in Europe. The new school of Paleobiology also 

 insists upon the close study of series of forms and rejects the 

 methods usually pursued by the neoembryologist, who, as a 

 rule, selects his objects of study and pursues his comparisons 

 upon the old basis of comparative anatomy and with but little 

 regard to the serial connections of forms. The importance of 

 studying the seriality in structure of the members of the same 

 group, those gradations, which lead from one variety to 

 another, one species to another, one genus to another, until 

 they may end in highly differentiated and often degraded off- 

 shoots, with as strange and unique developments as they have 

 adult characters, seems not, as yet, to have attracted the atten- 

 tion of the students of development among recent animals as 

 it has that of paleobiologists. The prevalent modes of study 

 'of living types has consequently led to noticing the phenomena 

 of omission of hereditary characters only in an isolated way, and 

 from the time of Balfour's " Comparative Embryology " these 

 omissions occurring in the embryo have been named abbrevia- 

 tions, shortenings and omissions of development, and various 

 attempts have been made to explain them upon more or less 

 general grounds of inference. Prof. Cope and the writer and 

 some other authors have been for a number of years publishing 

 observations upon this class of phenomena under the title of 

 the law of acceleration, asserting that in following out the his- 

 tory of series in time, or of existing series in structure, there 

 was observable a constant tendency in the successive members 

 (species, genera, etc.) of the same natural group to inherit the 

 characters of their ancestors at earlier stages than those in 

 which they appeared in these ancestors. That as a corollary 

 of this tendency, the terminal forms eventually skipped or 

 omitted certain ancestral characteristics, which were present in 

 the young of the preceding or normal forms of the same series, 

 and also in the adult stages of development of more remote 

 ancestors of the same genetic stock or series. This law has 

 since been independently rediscovered by several other nat- 

 uralists, notably Wiirtemburger in Germany, and Buckman in 

 England. The writer has lately christened this as the law of 



