Royal Institution. 61 



there is an analogy only when the former event occurs prematurely 

 through the influence of destroying conditions. But in their absence, 

 an individual after its period of vitality has been completed must 

 necessarily die ; whereas we have no right to assume that such would 

 be the fate of a species so circumstanced, since in every case where 

 we can either geologically or geographically trace a species to its 

 local or general extinction, we can connect the fact of its disappearance 

 with the evidences of physical changes. 



[The Lecturer illustrated these points by diagrams and special de- 

 monstrations, selecting for explanation two local cases, the one marine 

 and the other freshwater ; the former taken from the geological 

 phsenomena of Culver Cliff and the neighbouring bays in the Isle of 

 Wight, of which a beautiful and original model had been communi- 

 cated by Capt. Ibbetson for the purpose, and the latter from his own 

 recent researches (unpublished) on the succession of organic remains 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 centralization 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 following 

 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 altotted to it according to its kind. 



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

 individuals resembling each other in certain constant characters, or 

 hold, in addition, the hypothesis (warranted, as might be shown from 

 experience and experiment), that between all the members of such an 

 assemblage there is the relationship of family, the relationship of 

 descent, and consequently that they are all the descendants of one 

 first stock or pi-otoplast — (how that protoplast appeared is not part 

 of the question) — is like the individual in so much as its relations to 

 time are unique : once destroyed, it never reappears. 



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

 individual, it is continued indefinitely so long as conditions favourable 

 to its diffusion and prosperity — that is to say, so long as conditions 

 favourable to the production and sustenance of the individual repre- 

 sentatives or elements are continued coincidently with its existence. 



[No amount of favouring conditions can recall a species once 

 destroyed. — On this conclusion, founded upon all facts hitherto 



