618 ZOOLOGY 



SECT. 



3. GEOLOGICAL DISTRIBUTION. 



In considering the distribution of animals in past time, we are 

 met at the outset with the difficulty that our knowledge of the 

 subject is, and must always remain, very imperfect and fragmentary. 

 With few exceptions, only calcined, silicified, or strongly chitinised 

 parts are preserved in the fossil state, so that whole classes of 

 animals are absolutely unknown in that condition, and of the 

 rest our whole information depends upon the more or less imperfect 

 skeleton. Moreover, it is only under very favourable circumstances 

 that even the hard parts are preserved ; the chances are usually 

 in favour of the animal being devoured or disintegrated before 

 there is time for it to be silted over with mud or sand. And, 

 lastly, many rocks have been so altered by the internal heat of 

 the earth as to destroy any organic remains they may once have 

 contained. Thus, while palaeontology furnishes us with the only 

 sure test of phylogenetic speculation, it is a test which, more often 

 than not, is incapable of application, owing to the extreme imper- 

 fection- of many parts of the geological record. 



It is in the oldest of the stratified rocks that this imperfection 

 is most severely felt. In the Laurentian period, forming the 

 base of the sedimentary series (see Vol. I., p. 7), no animal or 

 vegetable remains are known. In certain Canadian serpentine 

 rocks belonging to this period there is found a remarkable structure 

 which, under the microscope, bears a certain resemblance to the 

 supplementary skeleton, with its canal-system, of an immense Fora- 

 minifer. On the assumption that it was the fossilised remains of 

 a member of this order, it was called Eozoon canadense, but later 

 researches seem to have shown conclusively that the supposed fossil 

 is of purely mineral origin. Radiolarians and Foraminifera have 

 been described from the Pre-Cambrian rocks of Brittany, but the 

 nature of the bodies in question has not yet been established 

 beyond dispute. 



There are thus no undoubted fossil animals until the Cam- 

 brian period, where many existing groups appear to start 

 suddenly into being. We find Radiolaria, Sponges, Graptolites, 

 Polyzoa, Brachiopoda, Edriasteroidea, Carpoidea, Asteroidea, 

 Chsetopoda (worm-tubes), Phyllocarida, Ostracoda, Trilobites, the 

 generalised Insects known as Palseodictyoptera, iso- and hetero- 

 myarian Pelecypods, Gastropods (Prosobranchs and Pteropods), 

 and tetrabranchiate Cephalopods (Orthoceras, &c.) all, it will be 

 noticed, marine forms, with the exception of Insects. 



Proceeding a stage onwards we find in the Silurian period, 

 in addition to the above groups, Foraminifera, Actinozoa (rugose 

 Corals), Ophiuroids, Echinoids, Crinoids, Cirripedes, Scorpions, 

 Eurypterida, Amphineura, Scaphopoda, Elasmobranchii, and 

 Ostracodermi. 



