144 The American Geologist. Soptombor, i896 
of the "Biogenetic Law."' According to tliis the development 
or ontogen}^ of each individual is only a short recaj)itulation 
of the long course of ancestral history (phylogeny) of the 
species or family in question. There must, therefore, also be 
chronological series of fossil embryonic types which would 
correspond with the different stages in the development of a 
subsequently existing form; indeed, the separate divisions of 
a genealogical tree must correspond essentially with the onto- 
genetic stages of a determined course of development. If the 
biogenetic law were correct, embryology would thus be in a 
position to reconstruct, at least approximately, the primitive 
forerunners of each group of plants and animals; and these 
types should, if they were capable of preservation, also lie 
buried in the rocks. 
If we consiilt paleontology, it shows that these surmises 
are by no means confirmed. There are, indeed, a gi'eat num- 
ber of fossil genera which retain throughout life the embry- 
onic, or, rather, the youthful characters of their existing 
allies, but it is only among the mammals, and to some extent 
among the reptiles, that I could name a complete series of 
forms following one another in time and belonging to the 
same line of development. The Eocene, Oligocene and. in 
part also, even the Miocene Mammalia, stand to their now 
existing allies, for the most part, in the relation of youthful 
forms, while they, almost without exception, exhibit at least 
some characters which are quickly passed through by their 
geologically younger successors in the embryonic or youthful 
stage. On the other hand they are, as a rule, destitute of the 
most striking peculiarities, such as antlers, bony processes, 
fusion of certain bones, reduction of the teeth or of separate 
parts of the skeleton ; and it is not till we study more closely 
a series of related genera of diiferent geological ages that we 
see how the ditferentiations and peculiarities of the existing 
representatives of any particular group have been gradually 
formed in course of time. But thus it is also possible to dis- 
cover, in most of the mammalian orders, a number of primi- 
tive characters, which, while the}'^ frequently occur united in 
the oldest representatives of the group in question, also usually 
correspond to an embryonic stage of one of its living 
members. 
