4 INTRODUCTION. 



together all ancient animals existing in the form of fossils. 

 These have been arranged stratigraphically, beginning 

 with the fossils in the oldest strata and ending with those 

 in the newest, or vice versa. For the purpose of historic 

 geological study or for strictly palaeontological research, 

 such collections are helpful. This arrangement has been, 

 indeed, the only one possible up to within a very recent 

 period. Even now there are naturalists who seriously 

 question whether any other method of arrangement is 

 possible. These maintain that our knowledge is not ade- 

 quate to warrant an attempt at a natural classification of 

 animals based upon genetic relationship. 



It must be borne in mind, however, that a vast amount 

 of material in the form of well proven facts has accumu- 

 lated since 1859, when Darwin's Origin of Species gave 

 a new directive impulse to biological research. This ma- 

 terial is found largely, it is true, in an almost infinite 

 number of papers in which isolated adult species are 

 figured and described. But notwithstanding this fact, it 

 is also true that of late years there has been a growing 

 tendency toward considering the life history of the indi- 

 vidual species described, and this study has led in turn 

 to an investigation into the life history of the group to 

 which the given species belonged. In this way the signi- 

 ficant correlation sexisting between the development of 

 the individual, known as ontogeny, and the development 

 of its class, or phylogeny, have been discovered. When 

 it is remembered that these correlations are proofs of 

 relationship or of descent from a common ancestor, then 

 one begins to realize how many trustworthy guides there 

 are, all pointing toward the desired goal. Our artificial 

 classifications of animals, therefore, are not due " so 

 much to insufficient knowledge of their early stages as 

 to insufficient attention to what is actually known and 

 published regarding them," as .Mr. Samuel H. Scudder 1 



lr The Butterflies of New England, I, 1889, p. viii. 



