166 .PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 7, 



semble each other, as well as those which gradually diverge, will 

 be placed in regular order ; and it will then be in my power to show, 

 to a certain extent, what the Bracliiopoda may do for the theory of 

 descent with modification. The difficulties to contend with in such 

 an investigation are, however, far greater than they are generally 

 supposed to be ; but it is quite evident, and my daily experience 

 (which, as far as the Bracliiopoda are concerned, exceeds twenty 

 years) teaches me, that the number of supposed distinct species has 

 been wonderfully exaggerated, and that a vast number of them are 

 in all probability, and in many cases certainly, mere modifications 

 in shape, or varieties in the Darwinian sense, which palaeontologists 

 will sooner or later be able to link together. Indeed, are we not con- 

 tinually puzzled to know how to find words or characters to distin- 

 guish our nearly related species ? Are not our supposed distinctions 

 often exceedingly uncertain ? and are we not generally far more pre- 

 occupied with the desire to find out trifling differences than to ex- 

 amine whether a closely related form might not be due to descent 

 with modification? To what extent variation has extended, our 

 knowledge will not enable us to determine ; and, therefore, we can- 

 not pretend to know whether a supposed new form is in reality the 

 result of a distinct and original creation. We may suppose it so, 

 with more or less reason, founding the supposition on the imperfect 

 state of the science, which leaves us no other alternative than to 

 consider new that which we cannot connect or identify with what is 

 already known. What, again, adds to the difficulty is the imperfect 

 knowledge we possess as to what is really the value of the term 

 "species," — for on that primary question naturalists are far from 

 unanimous*. I am of opinion that there has never been a total 

 extinction of life since its first appearance, and consequently no 

 sudden renewal of the entire fauna at any geological period. It is 

 likewise quite certain that species have gradually and continually 

 become extinct from the date when the first form was created, and 

 that perhaps new species have appeared in the same ratio, to suit the 

 different conditions of sea and land in which they had to exist f. It 

 is also highly probable, if not quite certain, that there never has 



* In 1860, Prof. Suess registered 1934 species of Brachiopoda ; and this is 

 far short of the total number that have been described ; but it would be most 

 erroneous to suppose that these 2000 or more names represent as many inde- 

 pendently created species, a large proportion being either synonyms, varieties, or 

 modifications in shape, of a limited number of original types. 



t M. Deshayes states that " it is an absolute palaontological demarcation 

 which serves to clearly separate the geological formations ; that at St. Cassian 

 riot a single species has been known to pass from the Triassic into the Jurassic 

 formation ; and that the same thing takes place between the five great series of 

 formations which he admits." (Bull. Soc. Geol. France, 2 ine ser. vol. xix. 

 p. 397, 1862.) I am sorry to be obliged to dissent from this view, which is 

 entirely in opposition to my experience ; I neither believe in the existence of 

 five absolute series of formations, or in any absolute palaeontological demarcation 

 between any geological systems that have been hitherto proposed, and for the 

 reasons above specified. On the contrary, I entirely concur with M. de Verneuil 

 while stating that he attaches less importance than do many geologists to those 

 geological and palgeontological divisions of the crust of the globe, which are 



