8 HYATT ON THE TERTIARY SPECIES 



of PI. levis, without which I could not have continued my work. The owners of the 

 Pits in Steinheim were uniformly kind and obliging, as were all the persons with 

 whom I came in contact at that place, and the accomplishment of my explorations ren- 

 dered easier and much facilitated by them. 



I. General Relations of the Series. 



The genealogical series illustrated on Plate 9, are constructed in accordance with facts 

 discussed in the chapter on the '' Descriptions of the series," but they also possess certain 

 peculiar characteristics of their own, which require explanation. 



A glance will indicate what the main assumption is, that all of the forms found in the 

 Pit Deposits are the direct descendants of four varieties of a species, which is taxon- 

 omically the normal form to which all the 2:)rimordial forms of the four series can 

 be referred. In other words this form, PI. levis, stands at the focus of all the affinities 

 of the four series, and is related to them in such a manner that we can only explain the 

 arrangement of the facts by supposing that this is the ancestral form from which they 

 sprang. The geological j)osition of PI. levis also justifies this conclusion with regard to all 

 the series, since it is a common form in the adjoining Tertiary rocks, as is admitted by all 

 authors. 



These series, having been the result of no preconceived plan of arrangement as far as 

 the author could judge, were considered to be approximately natural, and were assumed to 

 be a reliable basis for working hypotheses, in spite of the fact, that no certain data with 

 regard to succession in time were obtainable, except in the case of the supposed ancestral 

 species. This assumption rests largely upon well known laws of heredity, such as these, 

 that an animal found to repeat the stages of another animal of a closely allied species 

 in the young, with the addition of new characteristics in the adult, may be considered to 

 be either a lineal descendant of that species, or of some form common to both ; that in such 

 cases as these, whether the forms or species occur mixed on the same level, or on different 

 levels, there is but one natural arrangement, which has been illustrated on Plate 9. 



Such an arrangement in a diverging series can also, by varying the primal norm or 

 starting point, be used to represent the relations of a brood of individuals, or a 

 species, or a number of species ; in fact it is precisely the same as Darwin's diagram of 

 lines diverging from a point of origin ; and after seventeen years of investigation I am 

 entirely unable to propose any fundamental improvement in this mode of presenting 

 natural affinities. It represents with the same accuracy the parallel succession of charac- 

 teristics of the individuals, and also the parallel reproduction of similar forms in varieties, 

 species and larger groups, having as approximately determined by their intermediate forms, 

 embryology and structure a common origin. It represents also the relationship in time of 

 groups upon different levels in geological history, and their parallelisms and differences. 

 It represents these relations equally well for retrogressive, or progressive series according 

 to the values we may assign to each line, and can be made to coincide with the true time 

 ratio or relation in time of all the forms. 



