8 ELEMENTS OF PALAEONTOLOGY 
The results of embryological inquiry have a most important bearing on 
palaeontology. Numerous fossil forms are known, which, in comparison 
with recent related organisms, exhibit embryonic, or at least larval or adol- 
escent characteristics. Examples of such persistent embryonic types are especi- 
ally common in vertebrates, for the reason that here the skeleton becomes 
ossified very early in life, and hence the immature stages of the recent can be 
direcily compared with adult fossil forms. Now, observation has shown that 
in most of the older fossil fishes and reptiles, the vertebral column never 
passed beyond an embryonic stage, but remained in a cartilaginous or 
incompletely ossified condition through life. The Palaeozoic amphibians 
(Stegocephalia) probably breathed by means of both gills and lungs through- 
out life, whereas most recent amphibians lose their gills comparatively early 
(Caducibranchia), and breathe wholly by lungs. Many fossil reptiles and 
mammals retain certain skeletal peculiarities permanently, while allied recent 
forms exhibit them only in embryonic stages. The construction and shape 
of the skull in most of the older fossil reptiles and mammals closely corre- 
sponds with that in embryoes of recent related types. In the oldest fossil 
artiodactyls the palm-bones are all completely separated, while in recent 
ruminants this division continues only during the embryonic stage, being 
followed by a fusion of the two median metapodals, together with a reduction 
of the laterals. Among invertebrates, also, fossil embryonic types are by no 
means uncommon. The Palaeozoic belinuridae find their counterpart in the 
larvae of the common Limulus ; many fossil sea-urchins are characterised by 
linear ambulacra, while recent related forms, although developing petaloid radii 
in the adult stage, pass through the linear phase during adolescence. Many 
fossil crinoids before maturity resemble the living genus Antedon ; and, 
according to Jackson, recent oysters and Pectens exhibit in their nepionic 
stages certain characters peculiar to Palaeozoic genera of mollusks. 
The so-called fossil generalised or comprehensive types, which unite in one and 
the same form characters which, in geologically later, or recent descendants, 
have become distributed among different genera and families, are in reality 
merely adolescent or immature types which have stopped short of the higher 
differentiation attained by their descendants. Generalised types always 
precede more highly specialised; and properties that were originally distri- 
butive among older forms are never reunited in geologically younger species 
or genera. ‘Trilobites, amphibians, and reptiles of the Palaeozoic and Mesozoic 
eras, and early Tertiary mammals belong almost exclusively to the category 
of generalised types. 
In certain groups of vertebrates, and especially of mammals (Ungulata, 
Carnivora), the chronological succession of genera is so closely paralleled by 
the successive stages of development in the life-history of their descendants, 
that to a certain extent the ontogeny of the individual is a representment of 
a long chronological series of fossil forms. This truth furnishes a strong 
foundation for the biogenetic law, enunciated in various terms by Geoffroy St. 
Hilaire, Serres, Meckel, Fritz Miiller, and others, and recently more precisely 
formulated by Haeckel, as follows :—The developmental history, or ontogeny of 
an individual is merely a short and simplified repetition or recapitulation of 
the slow (perhaps extending over thousands of years) process of evolution of 
the species and of the whole branch. 
The biogenetic law has since been found to hold true not only.for verte- 
