5o 



GENESIS OF MAN. 



and this creature may therefore be regarded as a living representa- 

 tive of the true Chordonium. 



The Amphioxus forms the ninth stage in the anthropogenetic 

 line, and furnishes the first link in the vertebrate chain. It is the 

 only known representative of the once great subdivision of verte- 

 brates called by Haeckel the Acrania, or skulless vertebrates. 10 Of 

 this wholly unique and extremely interesting creature sufficient 

 mention has already been made. 



The tenth stage is that of the Monorhina, or Cyclostoma, which 

 have for their best-known representative the Petromyzon or lamprey. 

 These arose out of the Acrania through a simple enlargement of 

 the anterior extremity of the spinal nerve and the differentiation of 

 the corresponding part of the chorda dorsalis into a rudimentary 

 cranium. The distinctive circular mouth-orifice Haeckel regards 

 as a mere adaptive character not present in the original progenitor 

 of the Craniota. 



The transition to the eleventh, the Selachian or primordial fish- 

 stage, took place through the formation of a pair of nostrils and a 

 pair of jaws out of the simple, circular mouth-orifice of the Cyclos- 

 toma. This transformation led to the Amphirhina, a branch of 

 the Craniota, systematically coordinate with the Monorhina, but 

 embracing all the rest of the vertebrate sub-kingdom. With the 

 higher fishes (Ganoides and Teleostei) human genealogy is not 

 immediately concerned, but only with the lowest sub-class, the 

 Selachii, whose present living representatives are few, and comprise 

 the sharks, rays, etc., but which formed, in the Devonian age, the 

 chief population of the waters of the globe, as their singular 

 heterocercal remains, found in the rocks of that period, abundantly 

 attest. 



The transition from the Selachians was next to the Dipneusta, 

 which constitute the twelfth stage. It was brought about by the 

 natural adaptation of the organs of the body to a partially terres- 

 trial existence. The swim-bladders were transformed into imper- 

 fect lungs, the nasal orifices, which in fishes have no communication 

 with the interior of the mouth, established such a communication, 

 and the single auricle of the heart divided into two, thus corre- 

 spondingly improving the circulation of the blood. The Dipneusta 



10 See note 9, page 41. 



