168 MEANING OF THE SYSTEM. 



see filled in future clays, when knowledge has increased, and per- 

 haps only when formations now beneath the sea have become known 

 to us. 



Imperfection of the Geological Record. After the foregoing dis- 

 cussion we may consider that the continuity of living organisms in 

 the successive periods of the earth's development and their close 

 relationship has been proved partly by geological and partly by 

 palseontological facts. The theory of descent, however, according to 

 which the natural system must be regarded as a genealogical tree, 

 requires still further proof. It requires proof of the presence of 

 numerous forms, transitional not only between the species now 

 existing and those in the more recent formations, but also between 

 the species in all those formations which have immediately succeeded 

 one another in point of time. The theory also demands proof that 

 forms connecting the different groups of plants and animals of the 

 present day have existed. The establishment and limitation of these 

 groups can, according to Darwin, only be explained by the extinction, 

 in the course of the earth's history, of numerous and intimately 

 connected species. Palaeontology is only able imperfectly to comply 

 with these demands ; for the numerous closely graduated series of 

 varieties which, according to the theory of selection, must have 

 existed, are, for the greater number of forms, entirely wanting in 

 the geological record. 



This want, however, which Darwin himself recognised as an 

 objection to his theory, loses its importance when we consider the 

 circumstances under which organic remains were generally deposited 

 in mud, and preserved for succeeding ages in a fossil form ; when 

 we recognise the facts which indicate the extraordinary incomplete- 

 ness of the geological record, and which show that the intermediate 

 forms must have been in part described as species. 



First of all we can only expect to find in deposits the remains of 

 those organisms which possessed a firm skeleton supporting the softer 

 parts of the body, since it is only the harder structures of the body, 

 such as the bones and teeth of "Vertebrates, the calcareous and 

 silicious shells of Molluscs and Rhizopods, the shells and spines of 

 Echinoderms, the chitinous skeleton of Arthropods, etc., which are 

 able to resist rapid decay, and to undergo gradual petrifaction. 

 Thus the geological record will fail to provide us with any account of 

 the numerous and principally low organisms which are not pro- 

 vided with firm skeletal structures. 



But also among those organisms which are capable of becoming 



