PAs 
AMERICAN NATURALIST 
Vou. XXXII. September, 1898. No. 381. 
A HALF-CENTURY OF EVOLUTION, WITH 
SPECIAL REFERENCE TO THE EFFECTS 
OF GEOLOGICAL CHANGES ON 
ANIMAL LIFE. 
ALPHEUS S. PACKARD. 
Onty a little less than fifty years have passed since the pub- 
lication of Darwin’s Origin of Species, and the general accept- 
ance by naturalists of the theory of descent. Since 1848 the 
sciences of embryology, cytology, and comparative anatomy 
based on embryology, or, as it is now called, morphology, have 
been placed on a firm foundation. It is but little over half a 
century since the uniformitarian views of Lyell were promul- 
gated. The cell doctrine was born in 1839; the view that 
protoplasm forms the basis of life was generally received forty 
years since ; fifty years ago the doctrine of the conservation 
of forces was worked out, and already by this time had the 
idea of the unity of nature dominated the world of science. 
On the fiftieth anniversary, therefore, of our Association it 
may not be out of place, during the hour before us, first, briefly 
1 Address of the Vice-President and Chairman of Section F, ZoGlogy, at the 
fiftieth anniver rsary meeting of the American Association for the Advancement 
of Science, Boston, August, 1898. 
