592 



GEOLOGICAL HISTORY. 



which succeeded and characterized subsequent time, have been dis- 

 cussed in the course of the General Observations on the Archaean, 

 Paleozoic, Mesozoic, and Cenozoic eras ; and the reader is here re- 

 ferred to pp. 160, 389, and 520, a recapitulation in this place being 

 unnecessary. 



3. PROGRESS OF LIFE. 



Several general principles connected with the progress of life have 

 been illustrated in the course of the preceding history. They are here 

 brought together in brief review. 1 



!The following are some of the Criteria of Rank among Animals: — 



(1.) Under any type, water-species are inferior to land-species: as the Seals to the ter- 

 restrial Carnivores; the water-articulates, or Worms and Crustaceans, to land-articu- 

 lates, or Spiders and Insects. 



(2.) Species of a tr.be bearing some of the characteristics of an inferior tribe or class 

 are inferior spi cies, and conversely. — Thus, Amphibians show their inferiority to True 

 Reptiles, in the young having gills, like Fishes: the early Thecodont Reptiles, inferiority 

 to the later, in having biconcave vertebrae, like Fishes; the Marsupials and Edentates, 

 inferiority to other Mammals, in having the sacrum consisting of only two united ver- 

 tebrae, as in most Reptiles. On the contrary, the Dinosaurs show their superiority to 

 other Saurians, in having the sacrum made of five (or six) vertebrae, as in the higher 

 Mammals. 



(3.) As a species in development passes through successive stages of progress, relative 

 grade in inferior species may often be determined by comparing their structures with these 

 embryonic stages. — As a many-jointed larve, without any distinction of thorax and 

 abdomen, is the young state of an Insect, therefore Myriapods, or Centipedes, which 

 have the same general form, are inferior to Insects. As a young living Gar has a ver- 

 tebrated caudal lobe (making an accessory upper lobe to the tail), which it loses on becom- 

 ing adult, therefore the older Ganoids, with vertebrated tails (or heterocercal), are inferior 

 to the later, in which the tails are not vertebrated (or are homocercal). As the young of a 

 Frog (a tadpole) has the tail and form of a Salamandrian, therefore the Salamandrians 

 are inferior to Frogs. As the number of segments in the young of Insects often exceeds 

 much that of the adult, therefore species of adult animals in which there is an exces- 

 sive number of segments (beyond the typical number) have in this a mark of inferiority; 

 and thus the Phyllopods and Trilobites among Crustaceans bear marks of inferiority, 

 the typical number of segments in the abdomen of a Crustacean being but seven, and 

 in the whole body twenty-one, — each pair of members corresponding to one, commen- 

 cing with the eyes as the anterior. 



Professor Agassiz has brought out and illustrated in his writings each of the above 

 criteria. 



(4.) Species having the largest number of distinct segments in the posterior part of the 

 body, or having the body posteriorly prolonged, are the inferior among those under any 

 type. —Shrimps and Lobsters are thus inferior to Crabs; Centipedes, to Insects; Sala- 

 mandrians, or tailed Batrachians, to the Frogs, or tailless Batrachians; Snakes, to Liz- 

 ards; the Ganoids with vertebrated tails, to those with non-vertebrated. It does not 

 follow on this principle that Frogs, although tailless, are superior to Lizards: for they 

 are of different types of structure. 



(5.) Species having the anterior part of the body most compacted or condensed in ar- 

 rangement, or having the largest part of the body contributing to the functions of the 

 head-extremity, are the superior, other things being equal. — Thus, Man stands at the 

 head of all Vertebrates, in having only the posterior limbs required for locomotion, the 

 anterior having higher uses ; and also in having the head most compacted in structure, 



