MENTAL LIFE 



and complex habits among them. These hereditary 

 habits are gradually converted into instincts by func- 

 tional adaptation and progressive heredity; and with the 

 further development of consciousness in the higher 

 mammals we have at last the appearance of reason. 

 The gradual unfolding of the mental life is accompanied 

 step by step with the advance of its anatomic organ, the 

 phronema in the cortex. Recent careful investigations 

 of the ontogeny and histology of the origin of mind (by 

 Flechsig, Hitzig, Edinger, Ziehen, Oscar Vogt, etc.) have 

 given us an interesting insight into the mysterious proc- 

 esses of its phylogeny. 



While the comparative anatomy of the cortex gives us 

 a good idea of the gradual historical development of the 

 mind in the higher classes of vertebrates, we get at the 

 same time from their fossilized remains positive indica- 

 tions as to the period of time in which this phylogenesis 

 has slowly taken place. The historical series in which 

 the classes of vertebrates have succeeded each other in 

 the great periods of the organic history of the earth is 

 directly demonstrated by their fossil remans — the real 

 commemorative medals of natural creation — and gives 

 us a most valuable record of the ancestral history of our 

 race and of the mind. The oldest strata that contain 

 vertebrate remains form the huge Silurian System, which 

 were, on the latest calculations, formed more than a 

 hundred million years ago. They contain a few fossil 

 fishes. In the succeeding Devonian System these are 

 followed by the dipneusta, transitional forms between 

 the fishes and the amphibia. The latter, the oldest four- 

 footed and five-toed vertebrates, appear in the Carbo- 

 niferous Period. They are succeeded in the Permian, 

 the next system, by the oldest amniotes, the primitive 

 reptiles (tocosauria). It is not until the next period (the 

 Triassic) that the oldest mammals are found, small 

 primitive monotremes (pantotheria), then marsupials in 



327 



