On the Central Nervous System of Ascidia dsc. 209 



distinct types further and further every year. It is now esta- 

 blished that there was an excessively sudden appearance of 

 vast numbers of forms in the Cambrian, or perhaps earlier, as 

 claimed by Professor Marcou and others. 



We have applied this specific statement as a generalization 

 to the history of smaller groups of fossils in several branches 

 of the animal kingdom and in many formations, and have 

 found that the sudden appearance of the smaller groups occurs 

 according to the same law. 



There is an obvious plasticity in the animals which first 

 make their appearance in any unoccupied field, or at the begin- 

 ning of any new formation, which reminds one of the plastic 

 nature of the most generalized type of Metazoa, the existing 

 Porifera. The generalized types, which always occur first in 

 time, exhibit like sponges exceptional capacity for adaptation 

 to the most varied requirements of the surroundings and all 

 of the conditions of the new period or habitat by the rapid de- 

 velopment of numbers of suitable and more highly specialized 

 forms, species and genera. 



The whole picture as presented by morphology, embryology, 

 and palaeontology favours the hypothesis we have previously 

 advanced in other papers, namely that the early geologic 

 history of animal life, like the early stages of development in 

 the embryo, was a more highly concentrated and accelerated 

 process in evolution than that which occurred at any subse- 

 quent period of the earth's history. 



The history of the Porifera and higher Protozoa suggests also 

 that the evolution of the Metazoa may have occurred more 

 rapidly than we can now calculate. One of the great errors 

 of the present day is the assumption that such changes and 

 transitions occurred slowly and gradually ; and it is evident 

 that this assumption is based almost wholly upon investiga- 

 tion of the more highly specialized animals, in which the 

 capacity for change may be reasonably considered as very 

 much less than in their more generalized and embryonic ances- 

 tral forms. 



XXIII. — Preliminary Communication on some Investigations 

 upon the Histological Btructure of the Central Nervous 

 System in the Ascidia and in Myxine glutinosa. By 

 Fridtjof Nansen *. 



It is proposed in the following pages to give only a mere 



* Translated by W. S. Dallas, F.L.S., from the ' Bergeus Museums 

 Aarsberetuiug for 1885,' pp. 55-78. 



