DURATION OF SPECIES. 353 



replenisliment in YQgQiaiioTi^imri passu with tliat great 

 operation in which physical force is stored up in vege- 

 table organisms, and by the expenditure or transfoiTaa- 

 tion of which their work, and that of all animals, is 

 carried on ? Whatever be the cause (if any there be) 

 which determines the decadence and death of species, 

 one cannot well believe that it is a consequence of a 

 dmiinution of their proper force by plant-development 

 and division ; for instance, that the sum of what is 

 called vital force in a full-grown tree is not greater, 

 instead of less, than that in the seedling, and in the 

 grove greater than in the single parental tree. This 

 power, if it be properly a force, is doubtless as truly 

 derived from the sunbeam as is the power which the 

 plant and animal expend in work. Here, then, is a 

 source of replenishment as lasting as the sun itself, 

 and a ground — so far as a supply of force is concerned 

 — for indefinite duration. For all that any one can 

 mean by the indefinite existence of species is, that they 

 may (for all that yet appears) continue while the exter- 

 nal conditions of their being or well-being continue. 



Perhaps, however, M. I^audin does not mean that 

 " evolutive force," or the force of vitality, is really 

 homologous with common physical force, but only 

 something which may be likened to it. In that case 

 the parallel has only a metaphorical value, and the rea- 

 son why variation must cease and species die out is 

 still to seek. In short, if that which continues the 

 series of individuals in propagation, whether like or 

 unlike the parents, be a force in the physical sense of 

 the term, then there is abundant provision in Nature 

 for its indefinite replenishment. If, rather, it be a 



