460 



DISTRIBUTION IN SPACE AND TIME 



dominant, and embraced a great variety of species, mostly be- 

 longing to extinct types. It is notable, however, that some of 

 the lowlier forms which lived at the beginning of the period, 

 e.g. the Tongue-Shells (Ltngula, &c., fig. 1314), have persisted 

 to the present day with but slight modification, so far as can be 

 judged from the shell alone. That these and other "persistent 

 types " should remain unmodified for vast periods 

 of time has been brought forward as an argu- 

 ment against the doctrine* of evolution. It is, on 

 the contrary, what might be expected to some- 

 times occur in animals devoid of relatively com- 

 plex adaptations to their surroundings. It was 

 also at one time positively stated that Lamp- 

 Shells, taken as a whole, afford no instance of 

 modification on evolutionary lines. Of late years, 

 however, thanks to the brilliant work of the American school of 

 geologists, we know that the evidence afforded by this group is 

 enough in itself to convince any candid naturalist that evolution 

 has been the guiding principle in the animal world. 



PALEOZOIC JOINTED -LIMBED ANIMALS (ARTHROPODA). All 

 the existing orders of Crustaceans (Crustacea) were represented 



Fig. 1314. A fossil 

 Tongue - Shell (Lingu- 

 lella), somewhat enlarged 



Fig- 1315- Upper Surfaces of three Trilobites. Olenus (left), Paradoxides (centre), Olenellus (right). Actual 



size indicated by the fractions. 



in the fauna of this epoch, except the Fork-footed Crustaceans 

 (Copepoda), which are of too delicate a nature to be preserved as 

 fossil, though they no doubt existed. It is noticeable that some 

 of the more primitive types made their appearance very early, 

 the contrary being true for the more specialized ones, such as 

 creatures of the prawn and crab kind. 



