Life of an Individual and the Duration of a Species. 131 



a single self-existing organism, however produced, or extend 

 it to the series of organisms, combined or independent, all 

 being products of a single ovum, its term of duration can be 

 abbreviated but not prolonged indefinitely, nor can the several 

 phases of its existence be repeated. Conditions may arrest 

 or hasten maturity, or prematurely destroy, but cannot, how- 

 ever favourable, reproduce a second maturity after decline 

 has commenced. 



Now, it is believed by many that a species (using the term 

 in the sense of an assemblage of individuals presenting cer- 

 tain constant 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 individual, — 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 pro- 

 portionately 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 characterise epochs or sections of 

 time, independent of all physical changes and modifying 

 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 disturb- 

 ances that affect them. Yet this latter inference, involving 

 as it does the supposition that the spread and continuity of 

 species in time is dependent upon physical influences, 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 

 is has two sources, the one direct, the other indirect. It is 

 not an induction, nor pretended to be, but an hypothesis as- 

 sumed through apparent 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 com- 



I 2 



