Reviews — Nicholson and Lydekker's Palaeontology. 83 



definition of a palaeontological species given by von-Zittel, viz. that 

 it comprises all those individuals, or remains of individuals, which 

 possess in common an assemblage of constant characters, and which 

 constitute collectively a distinctly circumscribed morphological series, 

 apart from all considerations relating to their range in time and in 

 space. 



The final chapter of the Introductory part treats of the Evolution 

 of Organic Types in time. The author points out that we are quite 

 ignorant of the animals and plants which constituted the first living 

 beings, and that we are likely to remain so ; that representatives of 

 all the invertebrate sub-kingdoms are present in the earliest fossil- 

 iferous deposits, and these as individuals are complex and highly 

 specialized in their structures, and we are bound to conclude that 

 these earliest known faunas must have been preceded by many others 

 altogether unknown to us, in which the gradual changes from the 

 primitive simpler types of life took place. Throughout the entire 

 geological succession we can note a continuous introduction of new 

 species, sometimes gradually, at others apparently very abruptly, 

 but the evidence of palaeontology points to the operation of some 

 general law of evolution, by which these new species have been 

 derived from pre-existing forms. This succession of life-forms is 

 further progressive in character, the evidence of palaeontology indi- 

 cating a distinct advance in complexity of organization of the later 

 as compared with the earlier morphological types. There is but 

 little chance of ascertaining from the rocks any knowledge of the 

 oi'der of the appearance of the different leading types of Invertebrates : 

 but we ought to be able to find traces of the first appearance of the 

 •higher classes of Vertebrates. The author remarks that there is no 

 direct palaeontological evidence which would certainly establish any 

 particular theory as to the precise mode in which this law of evolu- 

 tion has been carried on, nor is the evidence from this source con- 

 clusive as to the theory of the origin of species by natural selection. 

 Whilst it is true that in a certain number of instances it has been 

 found possible to connect two different specific types by means of 

 a long series of intermediate links, as a general rule, the known 

 transitional forms between allied groups are few in number. " It 

 cannot be doubted, therefore, that palaeontology has, so far, to a large 

 extent, failed to bring forward the numerous and closely graduated 

 series of intermediate forms which must at one time have existed, 

 supposing ' natural selection ' to be the sole agent in the origination 

 of new species. The absence of a sufficient number of such transi- 

 tional forms, and the insufficient connection between such as are 

 known to exist, may doubtless be in part explained by the known 

 ' imperfection of the geological record ' ; but this does not appear to 

 offer an adequate solution of the difficulty. The theory of the 

 ' origin of species by means of natural selection,' as elaborated by 

 the master-mind of Darwin, constitutes nevertheless an invaluable, 

 indeed, an indispensable guide in all branches of palaeontological 

 research." 



The greater part of the first volume treats of the palaeontology of 



