Pahfontology and the Biogenetic Law. — von Zittel. 141 
As Darwin forcibly insisted, the doctrine of descent depends 
in no small degree on paheontological facts. Thus, the great 
similarity of the fossils occuringin strata immediatel}^ super- 
posed one on another, e. g., the brachiopods, ammonites, and 
other molluscs, had made it difHcult for geologists to deter- 
mine the age of sedimentary rocks. In recent years a great 
number of closely-allied species have been traced through 
several superposed beds, stages or divisions of formations, 
their exact morphological relationships have been studied in 
the most careful manner, and thus the probability at least has 
been established, that we are liere dealing with a genealogical 
sequence of blood-relations. To be sure these do not as a rule 
form complete chains, wherein mutation is linked with muta- 
tion and species with species. They are rather discontinuous 
series, of which all the members change in a definite direction 
and obviously form steps in a line of development, which 
culminates in the last extinct or still-existing representatives. 
Among the better-known series of forms I will only refer to a 
few. The succession of genera, which leads from Hyracother- 
iitm — perhaps, indeed, from the five-toed Phenacodus — through 
Paloplotherium, Anchilophus, Anchitheriitm, and Hijyparion, 
to the single-toed horse, forms one of the most quoted and 
most beautiful examples of phylogenetic development. No 
less complete a series is presented by the genealogical tree of 
the camels, which already appear during the Eocene in North 
America, spread there in the Miocene and Pliocene, and then 
first emigrate to the old world. The pigs, also, and the Oreo- 
dontidffi, Anoplotheriidse, Tragulidje, and the ruminants 
studied in so masterly a manner by Riitimeyer, afford us more 
or less complete genealogical series, beside which may be 
ranged the crocodiles among reptiles, the Amioidei and Phy- 
sostomi among fishes. If we glance over these phylogenetic 
series, we observe that the final terms are almost always dis- 
tinguished from their predecessors by a more pronounced 
and distinctive differentiation ; and since we are accustomed 
to assign a higher rank to a specialised organism in which 
every function is performed by a special arrangement, 
than to a creature which performs its functions with few and 
less complicated parts, phylogenetic development, as a rule, 
mplies also progression and perfection. For the existence of 
