632 THE AMERICAN NATURALIST. [VOL XXXII. 
for the supposition that the chromatin in the nucleus of 
reproductive cells is the bearer of heredity. The theory has 
already led to prolonged discussions and opened up new lines 
of work in cytology and embryology. 
g. The subject of instinct, discussed both by morphologists 
and psychologists, particularly by Lloyd Morgan, has come 
to the front, while mental evolution has been discussed by 
Romanes and others. 
With all these theories before us, these currents and 
counter currents in evolutional thought bearing us rapidly 
along, at times perhaps carrying us somewhat out of our 
depth, the conclusion of the whole matter is that in the 
present state of zodlogy it will be wise to suspend our judg- 
ment on many theoretical matters, to wait for more light, and 
to confine our attention meanwhile to the observation and 
registration of facts, to careful experiments, and to repeated 
tests of mere theoretical assumptions. 
Meanwhile we may congratulate ourselves that we have been 
born and permitted to labor in this nineteenth century, the 
century which in zodlogical science has given us the best years 
of Lamarck’s life, a Cuvier, a Darwin, a von Baer, an Owen, 
an Agassiz, a Haeckel, a Spencer, and a Huxley — the founders 
of modern zodlogy—who have sketched out the grander 
features of our science so completely that it will, perhaps, be 
the work of many coming years to fill in the details. 
GEOLOGICAL CAUSES OF VARIATION AND OF THE EXTINCTION 
AND RENEWAL OF SPECIES. 
The most immediate and efficient cause of variation appears 
to be changes of environment or of the physical conditions of 
existence. These, besides the agencies of gravity, electricity, 
of the atmosphere, light, heat, cold, food, etc., comprise g€0- 
logical changes or revolutions in the topography of the earth’s 
surface at different periods. The latter causes appear to have 
had much to do with the process of extinction and renewal of 
plants and animals, 
