Life of an Individual and the Duration of a Species. 133 



[The lecturer illustrated these points by diagrams and 

 special demonstrations, selecting for explanation two local 

 cases, the one marine and the other fresh water ; the former 

 taken from the geological phenomena of Culver cliff and the 

 neighbouring bays in the Isle of Wight, of which a beautiful 

 and original model had been communicated by Captain Ibbet- 

 son for the purpose ; and the latter from his own recent re- 

 searches (unpublished) on the succession of organic remains 

 in the Purbeck strata of Dorsetshire, conducted as part of 

 the labours of the Geological Survey of Great Britain.] 



The second and more indirect source of the notion of the 

 life of a species may be traced in apparent analogies, half- 

 perceived, between the centralisation of generic groups in 

 time and space, and the limited duration of both species and 

 individual. But in this case ideas are compared which are 

 altogether and essentially distinct. 



The nature of this distinction is expressed among the fol- 

 lowing propositions, in which an attempt is made to contrast 

 the respective relations of individual^ species, and genus to 

 Geological time and Geographical space. 



A. The individual, whether we restrict the word to the 

 single organism, however produced — or extend it to the series 

 of organisms, combined or independent, all being products 

 of a single ovum — has but a limited and unique existence in 

 time, which, short as it must be, can be shortened by the 

 influence of unfavourable conditions, but which no combination 

 of favouring circumstances can prolong beyond the term of 

 life allotted to it according to its kind. 



B. The species, whether we restrict the term to assemblages 

 of individuals resembling each other in certain constant cha- 

 racters, or hold, in addition, the hypothesis (warranted, as 

 might be shewn from experience and experiment), that be- 

 tween all the members of such an assemblage there is the 

 relationship of family, the relationship of descent, and con- 

 sequently that they are all the descendents of one first stock 

 or protoplast — (how that protoplast appeared is not part of 

 the question) — is like the individual, in so much as its re- 

 lations to time are unique: once destroyed, it never reappears. 



But (and this is the point of the view now advocated), 



