193 GEOLOGICAL DISTRIBUTION. 



had the notion that the ammonites were restricted to the Mesozoic 

 era been engrafted on the minds of paleontologists ; but now, sev- 

 eral individuals, belonging to two or more species, have been ob- 

 tained from the same deposits, and one closely related form has 

 been quite recently described from the nearly equivalent deposits 

 of Texas.'' M. Barrande has laid great stress upon the sudden ap- 

 pearance, side by side, and in the full plenitude of their characters, 

 of the more distinctive genera of cephalopods (Orthoceras, Cyrto- 

 ceras. Nautilus, Trochoceras, Bathmpceras) in the first aspect of 

 his second Silurian fauna (Lower Silurian of geologists generally), 

 and their complete absence from the Primordial Zone (Cambrian) ; 

 but we have seen that at least two of the genera, Orthoceras and 

 Cyrtoceras, have since been traced back to the earlier formation. 

 Almost every chapter in geological history indicates some such case 

 of antedating, and proves to the paleontologist how very cautious 

 he should be in his limitation to time of particular groups of or- 

 ganisms. It cannot be expected that in any portion of the earth's 

 surface will there ever be found a complete sequence of the geologi- 

 cal formations, nor can we hope satisfactorily to bridge over in all 

 cases the gaps that occur in one' locality with the deposits found 

 in another. The unequal period of time during which land areas 

 have been laid dry or been kept submerged beneath water, in con- 

 junction with the devastating effects of denudation, render such a 

 complete restoration of the series impossible, and as long as this is 

 so, the work of the paleontologist must inevitably be riddled with 

 "breaks" in the geological record. It is surprising, in view of 

 these facts, which are too obvious to be overlooked, with what 

 tenacity some paleontologists insist upon absolute limitation of 

 species, or groups of species, and how slow they are to accept any 

 new facts bearing upon distribution in time that might in any 

 way disturb the harmony of their preconceived notions. There is 

 perhaps no more patent fact in the liistory of the physical de- 

 velopment of our planet than the imperfection of the geological 

 record,, the full realisation of which could not fail to dispel many 

 of the singular notions that still prevail relative to the support 

 which paleontology brings to the doctrine of evolution. 



It has been objected that, in assuming the universality of breaks, 

 we are drawing" largely upon our fancy, and that conditions which 

 do not exist in fact are arranged to suit the views of the evolution- 



