SUCCESSION AND DEVELOPMENT OF ANIMALS. 173 



taneously the embryonic growth of the higher Ento- 

 mostraca, such as Limulus, and that of the highest order 

 of the class, 1 when it will appear, that, as the former 

 recall in early life the form and character of the Trilobites, 

 so does the young Crab passing through the form of the 

 Isopods, and that of the Macrouran Decapod before it 

 assumes its typical form as Brachyuran, recall the well- 

 known succession of Crustacea through the geological 

 middle ages and the Tertiary periods to the present day. 

 The early appearance of Scorpions, in the Carboniferous 

 period, is probably also a fact to the point, if, as I have 

 attempted to show, Arachnoids may be considered as 

 exemplifying the chrysalis stage of development of 

 Insects ; 2 but, for reasons already stated, (Sect. XXIV.), 

 it is hardly possible to take Insects into consideration in 

 these inquiries. 



In my researches upon fossil Fishes, 3 I have pointed 

 out at length the embryonic character of the oldest fishes ; 

 but much remains to be done in that direction. The 

 only fact of importance I have learned of late is that the 

 young Lepidosteus, long after it has been hatched, ex- 

 hibits, in the form of its tail, characters hitherto only 

 known among the fossil fishes of the Devonian system. 4 

 It is to be hoped that the embryology of the Crocodile 

 will throw some light upon the succession of the gigantic 

 Keptiles of the middle geological ages, as I shall show 

 that the embryology of Turtles throws light upon the 

 fossil Chelonians. 5 It is already plain that the embryonic 

 changes of Batrachians coincide with what is known of 



1 AGASSIZ (L.,) Twelve Lectures, * AGASSIZ (L.,) Lake Superior, etc., 

 etc., p. 68. p. 254. 



2 Classif. of Insects, q. a., page 5 See ray Contributions to the Nat. 

 128. Hist, of the United States, vol. i, pp. 



3 Poiss. fossiles, q. a,, p. 81. 290, 303 and 386. 



