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 

 directly 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- 



