256 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



the young animal, become welded later in the adult 

 into a cannon-bone caused by the fusion of two 

 metapods. We know among the early Ruminants 

 genera such as the Pcebroiherium among the Cameli- 

 dae, and the Gelocus among the Tragulidae, in which 

 the metacarpal and metatarsal bones remain dis- 

 tinct in the adult stage. 



We may also quote a few persistent embryonic 

 types among the Invertebrates. Certain palaeozoic 

 Belinuridae, the Prestwichia, strangely resemble 

 the young larvae of the existing Limulus. The 

 Pentacrina larva of our Antedon, is very comparable 

 to many fossil Crinoids. The early Urchins with 

 linear ambulacra retain in the adult state the transi- 

 tory stage through which pass the more recent 

 Urchins with petaloi'd ambulacra. Lastly, among 

 Brachiopods, Baecker has shown that each stage 

 of growth of the brachial supports in the modern 

 Terebratulidae corresponds to some genus of fossil 

 Terebratula. 



But it must be very clearly stated that these 

 examples of representation in fossil adult species 

 of the embryonic, or more correctly, of the youthful 

 characteristics of existing animals, cannot be general- 

 ized, and that they remain up till now in the state 

 of exceptional facts. 



A second mode of investigation, more direct 

 and more sure, consists in studying the individual 

 evolution of the fossils themselves, from their early, 

 if not their embryonic, state to that of their adult 

 form. Unfortunately it is confronted in practice, 

 in most cases at least, by almost insurmountable 

 material difficulties, proceeding from the dearth 



