1872.] 191 | [Maack. 
whole of the hereditary influences existing in the creature and tend- 
ing to determine its mental constitution, have had a large part in the 
work. It was the intellectual needs which gave the hand. In get- 
ting the hand, a new attitude was forced on the body, and with this 
change, the result of intellectual needs, came a new series of selec- 
tive actions. The inherent possibilities of intellectual development 
have been the guiding agents in this series of changes. Selection 
has condemned the failures, and so strengthened the successes in the 
- struggle for better things. 
Dr. G. A. Maack gave an account of the geology of the 
Isthmus of Choco, of Darien and of Panama, which he sur- 
veyed during the year 1871, in behalf of the United States 
Government. He exhibited a large number of photographs 
and lithological specimens to the Society, and presented the 
latter collection to the Museum. 
Dr. Maack gave a detailed description of the geological 
structure of both the Atlantic and Pacific sides of the Isth- 
mus. He pointed out that the mountainous system in the 
Province of Choco, near Limon Bay, which opens on Cupica 
Bay, has a geological character different from that of the 
Tuyra and of the Chucunaque regions in the Province of 
Darien, while the latter mountains differ by their parallel 
arrangement, as well as by their petrographical composition, 
from those between Panama and Aspinwall. Consequently 
the whole Isthmus consists of three systems differmg from 
one another. The rocks of these three systems belong to the 
Tertiary period, and are of a volcanic character. They dif 
fer in this respect, that the mountains of the Province of 
Choco consist principally of propylite, the oldest tertiary 
volcanic rock, while the mountain ranges in the Province of 
Darien are formed of andesite, and the various peaks and 
domes between Panama and Aspinwall of trachyte and dol- 
erite. 
Dr. Maack called attention to the fact that the San Blas 
Cordillera on the Atlantic side, which lies between the Prov- 
inces of Panama and of Darien, consists of granite and sye- 
