CAMBRIAN FAUNA. 139 



cerned, the obstacle is more apparent than real, for, as has already 

 been insisted upon, it is impossible to determine how far back be- 

 yond the Cambrian, or first unequivocally fossiliferous formation, 

 life may have already existed, and, consequently, to what very 

 ancient period the ancestry of the moUuscan type may extend. As 

 a matter of fact, the most ancient mollusk, the Lingula, or Lingu- 

 Iclla, is almost the lowest in structure of any with which we are 

 acquainted, and if in the rock deposits we meet with its remains 

 but barely antedating those of the very much more highly organ- 

 ised Orthoceras, we have yet strong grounds for concluding that its 

 first appearance was very much earlier, only that, through the gen- 

 eral obliteration of all remains in the preceding geological period, 

 direct evidence to that effect has been lost. As to the other objec- 

 tion, that the predominant forms persisting at any given epoch 

 should be those whose structure manifests the highest development, 

 it may be remarked that the evolutionary force requires no such 

 result as the outcome of its operative action. It is among such 

 forms as, in their mutual relations to their surroundings, whatever 

 these may be, are best adapted or fitted for combatting the nu- 

 merous elements that constantly interpose themselves in the path 

 of existence, that we must look for examples of greatest persistence 

 and development — for the survivors in the struggle for existence. 

 Hence, while the highest developed forms in any given series of 

 animals will present themselves in or about the period most re- 

 moved from the birth of that series, yet it need not follow that 

 the higher series will ultimately outlive, or even predominate over, 

 the representatives of a lower parallel series of the same class of 

 animals, whose fitness for struggling in the battle for existence is 

 not infrequently vastly superior to that of the higher class. We 

 need not be, therefore, surprised at finding, in a given class of ani- 

 mals, some of the more perfect forms disappearing from the world's 

 horizon before the less perfect, and these last, consequently, the 

 survivors in the general battle for life. But, while no general law 

 can be formulated regarding tlie disappearance, as conditioned by 

 the degree of perfection, of the various series of a given class of 

 animals, or, regarding their relative development in any one period 

 of the earth's history, the law of appearance or succession already 

 stated — i. e., the introduction of lower forms before those of a 

 higher order — can very generally be maintained. 



