160 EVOLUTION OF LIFE. 
image, and special arrangements for the accommodation 
ofthe eye to distances. By glancing, therefore, at different 
animals, we see the eye in different stages of perfection. 
But the gradual development of the eye of man offers a 
series of transitional stages, which are permanently repre- 
sented by the eyes of the lower animals. Now, if the eye 
has gradually been perfected through the Survival of the 
Fittest, we understand why the transitional stages in the 
development of the eye of man have permanent represent- 
atives in the eyes of the lower animals; but if the eye has 
been created for the purpose of seeing, we neither under- 
stand why such an extraordinary method is adopted in its 
formation nor why it is not a perfect optical instrument. 
The same reasoning will apply to the human ear as well 
as to the eye, they both beginning as depressions in the 
skin, which later, closing, form the primitive eye and ear 
vesicles. 
RESUME. 
We tried to show in the chapters on Zoology, Botany, 
Geology, and Embryology, that the structure of plants and 
animals, their petrified remains, and their manner of develop- 
ment, are explained by supposing that life has evolved, that 
there has been a gradual development of the higher forms 
of life from the lower, accompanied here and there by a 
retrograding metamorphosis. In the early part of this 
chapter we called attention to the facts of Geographical 
Distribution not being consistent with a theory that sup- 
posed plants and animals had been created for special 
localities, but that they could be explained by supposing 
that life migrated from place to place, being more or less 
modified from time to time by the new conditions of exist- 
ence, natural barriers being often the cause of the great 
difference exhibited by plants and animals living under 
similar conditions. The conclusion of an Evolution of Life, 
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