DURATION OF SPECIES. 229 



currents which affect the temperature of a region, or by the 

 destruction of their food through climatic changes; but 

 when races w T ane and die out without any apparent change 

 in external conditions (just as individuals appear, grow up 

 to maturity, and then fade away), we are driven to some 

 such conclusion as the limited duration of specific force. 

 And if species thus depart without the operation of physical 

 causes, we are compelled to accept the converse, that they 

 may also make their appearance independently of the influ- 

 ences of those external conditions on which the Transmu- 

 tationists have based so much of their hypothesis. 



Nor is it the narrow circle of species alone, but the larger 

 groups and families seem also to have had a similar limit 

 assigned to their duration. The graptolites of siluria, the 

 paleozoic trilobites and eurypterites, the carboniferous sigil- 

 larise and lepidodendra, the ammonites of the oolite, the 

 enaliosaurs and dinosaurs of the same epoch, and the palaBO- 

 theres of the tertiary, all have had their beginning, their 

 culmination, in individual bulk and specific variety, their 

 declension and decay ; and this, be it observed, under no 

 phases of external conditions that geology can determine, 

 but apparently in obedience to some law of structural evolu- 

 tion which runs its course within a definite period. The 

 whole system of life, vegetable and animal, appears but to 

 be a pre-arranged series of typical ideas, each to be realised 

 at a certain period and Avithin certain limits of variation, 

 and when once realised to become passive for ever. The 

 realisation of these creative ideas must of course be accom- 

 panied by a thousand co-relative circumstances, and the 

 great caution of philosophy should be to avoid confounding 

 concomitants with causes, or mistaking mere ordinal arrange- 

 ment for sequential connection. 



