MORPHOLOGICAL DIFFERENTIATION. 49 



logical whole, and their several metamorphoses are so ordered 

 and related to one another that thej constitute members of a 

 mutually dependent commonalty. 



The Metazoa are the only animals which fall under com- 

 mon observation, and have therefore been known from the 

 earliest times. All the higher languages possess general 

 names equivalent to our beast, bird, reptile, fish, insect, and 

 worm ; and this sliows the very early perception of the fact 

 bhat, notwithstanding the wonderful diversity of animal forms, 

 they are modeled upon comparatively few great types. 



In the middle of the last century the founder of modern 

 Taxonomy, Linnaeus, distinguished animals into Mammalia^ 

 Aves, Amphibia, Pisces, Insecta, and Vermes, that is to say, 

 he converted common-sense into science by defining and giv- 

 ing precision to the rough distinctions arrived at by ordinary 

 observation. 



At the end of the centurv, Lamarck made a most impor- 

 tant advance in general morphology, by pointing out that 

 mammals, birds, reptiles, and fishes, are formed upon one type 

 or comtnon plan, the essential character of which is the pos- 

 session of a spinal column, interposed between a cerebro-spi- 

 nal and a visceral cavity ; and that in no other animals is the 

 same plan of construction to be discerned. Hence he drew a 

 broad distinction between the former and the latter, as the 

 Vertebrata and the Invertebrata. But the advance of 

 knowledge respecting the structure of invertebrated animals, 

 due chiefly to Swammerdam, Trembley, Reaumur, Peyssonel, 

 Goeze, Roesel, Ellis, Fabricius, O. F. Miilh'r, Lyonet, Pallas, 

 and Cuvier, speedily proved that the Invertehrata are not 

 framed upon one fundamental plan, but upon several ; and, 

 in 1795, Cuvier ^ showed that, at fewest, three morphological 

 types, as distinct from one another as they are from that of 

 the vertebrated animals, are distinguishable among the In- 

 vertehrata. These he named — I. Mollusques ; II. Insectes et 

 Vers ; III. Zoophytes. In the " Res^ne animal " (1816), those 

 terms are \^dX\mz^A, Anhnalia 3folh(Sca,Articulata,ii\\diIia- 

 diata. Thus, says Cuvier : " It will be found that there ex- 

 ist four principal forms, four general plans, if it may thus be 

 expressed, on which all animals appear to have been modeled ; 

 and the ulterior divisions of which, under whatever title natu- 

 ralists may have designated them, are merely slight modifica- 

 tions, founded on the development or addition of certain parts. 



* Tableau ^lementaire de I'Histoire des Animaux. An vi. 



