140 GEOLOGICAL DISTRIBUTION. 



An objection to the evolutionary doctrine, similar to that which 

 has been drawn from the distribution of the Mollusca, is also fur- 

 nished by the articulated animals, or, more particularly, by the class 

 of the Crustacea. The members of this class boast of a lineage, as 

 far as has yet been determined, very nearly, if not fully, as ancient 

 as that of the Mollusca, one extending back to the earliest Cambrian 

 period. But, while the most ancient mollusks with which we are 

 acquainted belong in great part to orders, families, and even genera, 

 whose representatives still flourish in the existing seas, the most 

 ancient crustaceans, or at least the majority of them, the Trilobita, 

 have long since become totally extinct ; hence the impossibility of 

 determining their true relationships. However uncertain or obscure 

 this relationship may be, whether it is with the Phyllopods, as 

 claimed by some, or, what is much more likely, with the Xiphosura 

 (king-crabs) and arachnids, as argued by others, there can be no 

 doubt, if homologies of structure can be relied upon, that the mem- 

 bers of this group of animals represent a high grade of structural 

 organisation, and especially if the period of their appearance is 

 taken into consideration. But here, just as in the case of the 

 Mollusca, we have the strongest evidence for concluding tliat their 

 earliest appearance dates far beyond the Cambrian period, as is 

 proved almost conclusively by the simultaneous appearance in the 

 oldest Cambrian strata of some of the simplest and most compli- 

 cated trilobitic forms, Agnostus and Paradoxides, which are at the 

 same time also among the smallest and the largest forms of the 

 entire order. A further evidence of the pre-Cambrian antiquity of 

 this group is furnished by the circumstance of the abundance in 

 which the earliest remains are found, an abundance which, though 

 perhaps not eqvial to that characteristic of the succeeding Silurian 

 and Devonian trilobitic faunas, is yet sufficient to impress a dis- 

 tmct individuality upon the fauna of the period. While with 

 the Cambrian trilobites we find associated other forms of crusta- 

 cean animals, such as the phyllopods (Hymenocaris) and ostracods 

 (Primitia, Leperditia), the highest members of the class, the Deca- 

 poda (crabs and lobsters), appear not to have been as yet evolved. 

 Indeed, it is not until the entire Silurian period and a considerable 

 portion of the Devonian are passed that we meet with an example 

 of the ten-legged order of crustaceans. Barely had these higher 

 forms asserted themselves on the field of life ere a decline in the 



