8 ELEMENTS OF PALEONTOLOGY 



characteristics. Examples of such primitive or emhryonic fypes 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 Paleozoic 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 skull in most of the older 

 fossil reptiles and mammals closely corresponds in form and structure with 

 that in embryos 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 Paleozoic 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 re- 

 semble the young of the living genus Antedon ; and, according to Jackson, 

 recent echinoids, oysters and pectens exhibit in their nepionic stages certain 

 characters peculiar to the adults of Paleozoic genera. 



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 primitive 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 Paleozoic 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 hiogenetic law, enunciated in various terms by GeofFroy St. 

 Hilaire, Serres, Meckel, Fritz Müller 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 brauch. 



The biogenetic law has since been found to hold true not only for verte- 

 brates, but also for invertebrates, including even wholly extinct types. In 

 ammonites, for instance, the primary or innermost whorls always difFer frora 

 the outer in their greater simplicity of suture, and in their lesser ornamenta- 



