PROGRESS OF LIFE. 593 



at the other ; and if Protophytes or any Algse, and Protozoans, com' 

 menced in the later part of the Azoic (which is shown to be pro^ 



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

 relative grade in inferior species may often be determined by comparing their struc- 

 tures toith these embryonic stages. — As a many-jointed larve without any distinc- 

 tion 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 vertebrated caudal lobe (making an accessory upper 

 lobe to the tail), which it loses on becoming 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 tad- 

 pole) 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 excessive 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 mem- 

 bers corresponding to one, commencing 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; Salamandrians, or tailed Batrachians, to the Frogs or tailless Batra- 

 chians; Snakes, to Lizards; the Ganoids with vertebrated tails, to those with 

 non-vertebrated. It does not follow on this principle that Frogs, although tail- 

 less, are superior to Lizards ; for they are of different types of structure. 



(5.) Sp>ecies having the anterior part of tlie body most compacted or condensed in 

 arrangement, 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 and brought into the least compass consistent with the 

 amount of brain. In the same manner, the Carnivores among the large Mam- 

 mals (Megasthenes) are superior to the Herbivores, the anterior limbs not having 

 locomotion as their sole use, and the head being more compacted and con- 

 densed for the size of brain. The highest Crabsj the Triangular or Maioids, are 

 superior in the same manner to the lower, and far more to the Lobster tribe and 

 other Macrourans ; descending in grade from the higher Crabs, the outer mouth- 

 organs become more and more separated from the mouth, and finally, in many 

 Macrourans, they have the form of feet, thus passing from the head-series to the 

 foot-series. Insects are on this principle superior as a class to Crustaceans, 

 although of so much less size. 



Condensation anteriorly and abbreviation posteriorly is the law of all pro- 

 gress in embryonic development, and also of relative rank among species of re- 

 lated groups. 



39 



