CuAi>. I. SUCCESSION AND DEVELOPMENT OF ANIMALS. 115 



1)0 suflicient as a l^asis for (similar comparisons. The class of Crustacea, on the 

 contrary, is very instructive in this i-cspect; but, to trace our comparisons through 

 the whole series, it is necessary that we should consider simultaneously tlie em- 

 bryonic growth of the liigher Entomostraca, such as Limulus, and tliat of the highest 

 order of the class,^ 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 Decapods, before it assumes its 

 typical form as Brachyouran, 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 j^eriod, is probably also a fact to the 

 point, if, as I have attempted to show, Arachnidians may be considered as exemphfy- 

 ing the chrysalis stage of development of Insects ; ^ but, for reasons already stated 

 (Sect. XXIV.) it is hardly possible to take Insects into consideration in these inqmries. 



In my researches upon fossil Fishes,^ I have pointed out at length the embr^-onic 

 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 3'oung Lepidosteus, 

 long after it has been hatched, exhibits in the form of its tail, characters, thus 

 far only known among the fossil fishes of the Devonian system.'* It is to be hoped, 

 that the embryology of the Ci'ocodile will throw some light upon the succession 

 of the gigantic Reptiles of the middle geological ages, as I shall show, that the 

 embryology of Turtles throws light upon the fossil Chelonians. It is already plain, 

 that the embryonic changes of Batrachians coincide with what is known of their 

 succession in past ages.^ The fossil Birds are too httle known, and the fossil 

 Mammalia'^ do not extend through a sufficiently long series of geological formations 

 to aflbrd many striking points of comparison ; yet, the characteristic peculiarities 

 of their extinct genera exhibit everywhere indications, that their living repi'esenta- 

 tives in early life resemble them more than they do their own parents. A minute 

 comparison of a young elephant, with any mastodon, will show this most fully, 

 not only in the peculiarities of their teeth, but even in the proportion of their 

 limbs, their toes, etc. 



It may, therefore, be considered as a general fact, very likely to be more fully 

 illustrated as investigations cover a wider ground, that the phases of development 

 of all living animals correspond to the order of succession of their extinct rejjre- 

 sentatives m past geological times. As for as this goes, the oldest representatives 



* Ac.ASSiz, (L.,) Twelve Leetiires, efe., p. 66. ' See the works, q. a., p. 82, note 3. 



* Classif. of Insects, q. a., ]>. 8.">. ' Ccv., Oss. foss., q. a. : also, Ar.ASSiz, (L.,) 



* Poiss. fossilcs, q. a., p. ii 1. Zoological Cliaracter of Young Jlanunalia, Proc. Am. 



* Agassiz, (L.,) Lake Superior, etc., p. 2J4. Ass. Adv. Sc, t'anitiridge, 1849, p. 85. 



