MORPHOLOGICAL DIFFERENTIATION. 49 



logical whole, and their several n,< -tamMi-plioses arc so ordered 

 and related to one another that ihry mnsiituii- members of a 

 mutually dependent commonalty . 



The Metazoa are the only animals which fall under com- 

 mon observation, and have therefore I.e. n kmmn frnm the 

 earliest times. All the higher languages possess general 

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

 worm ; and this shows the very early j . f the fact 



that, notwithstanding the wonderful diversity <*t animal forms, 

 tlu>y are modeled upon comparatively few great types. 



In the middle of the last century the founder of modern 

 Taxonomy, Linnreus, 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 century, 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 conynon plan, the essential character of which is tin* pos- 

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

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

 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. Hut the advance of 

 knowledge respecting the structure of invorlebrated animals, 

 due chiefly to Swammerdam, Trembley, H&iiiiniir, Peyss. 

 Goeze, Roesel, Ellis, Fabricius, O. F. MnlW, Lyonet, Pallas, 

 and Cuvier, speedily proved that the Invertebrate are not 

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

 in 1795, Cuvier l 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 / 

 vertebrata. These he named I. Mollusques ; II. Insectes et 

 Vers ; III. Zoophytes. In the " Rt\rn' animal " (1816), those 

 terms are Latinized, Animalia Molhisca)ArticulcUa,*xi<iRa- 

 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 b 

 and the ulterior divisions of which, under \\ hati-xvr titlr natu- 

 ralists may have designated them, are more 1 modifica- 

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



* Tableau &<$mentaire de PHistoire des Animaur. An vi. 

 3 



