60 Royal Institution. 



characters in common, and derived from one original protoplast or 

 stock) passes through a series of phases comparable with those which 

 succeed each other in definite order during the life of a single indivi- 

 dual, — that it has its epochs of origin, of maturity, of decline and of 

 extinction, dependent upon the laws of an inherent vitality. 



If this notion be true, the theory of Geology will be proportionately 

 affected ; since in this case the duration of species must be regarded 

 as only influenced, not determined, by the physical conditions among 

 which they are placed ; — and, thus, species should characterize epochs 

 or sections of time, independent of all physical changes and modify- 

 ing influences short of those which are absolutely destructive. Now, 

 geological epochs, as at present understood, are defined by peculiar 

 assemblages of species, and the amount of change in the organic 

 contents of proximate formations or strata is usually accepted as a 

 measure of the extent of the disturbances that affect them. Yet this 

 latter inference, involving as it does the supposition that the spread 

 and continuity of species in time are dependent upon physical influ- 

 ences, is adverse to the notion of a Life of a Species as stated above. 



If we seek for the origin of this notion, we shall find that it has 

 two sources ; the one direct, the other indirect. It is not an induc- 

 tion, nor pretended to be, but an hypothesis assumed through appa- 

 rent analogies. Its first and principal source may be discovered in 

 the comparison suggested by certain necessary phases in the duration 

 of the species with others in the life of an individual, such as each has 

 its commencement, and each has its cessation. Geological research 

 has made known to us, that prior to certain points in time certain 

 species did not exist, and that after certain points in time certain 

 species ceased to be. The commencement of a species has been com- 

 pared with Birth, the extinction with Death. Again, many species 

 can be shown to have had an epoch of maximum development in 

 time. This has been compared with the maturity of the individual. 



Between the birth of an individual and the commencement of a 

 species in the first appearance of its protoplast, the analogy is more 

 apparent than real. We know how the former phaenomenon takes 

 place, but we have no knowledge of the latter. 



Between the maturity of the individual and the maximum develop- 

 ment of a species there is no true analogy, since the latter can easily 

 be proved to be entirely dependent on the combination of favouring 

 conditions, and during the period of duration of a sjjecies there may 

 be two or more epochs of great or even equal development, and two 

 or more epochs of decline alternating with epochs of prosperity. The 

 epoch of maximum of a species may also occur during any period in 

 its history short of the first stage. Geological and geographical re- 

 search equally show that the flourishing of a species is invariably 

 coincident with the presence of favouring and its decline with that of 

 unfavourable conditions. Hence there is no analogy between the 

 single and definite phase of maturity of the individual, and the varia- 

 ble and sometimes often-repeated epochs of luxuriant development in 

 the duration of a species. 



Between the death of the individual and the extinction of a species 



