362 THE HISTORY OF CREATION. 
of human ancestors sketched in a late chapter gives an ap- 
proximate representation. 
It is true that this conception generally greatly offends 
most persons on their first becoming acquainted with the 
Theory of Development, because more than all others it 
most strongly contradicts the traditional and mythological 
ideas, and the prejudices which have been held sacred for 
thousands of years. But like all other functions of organ- 
isms, the human soul must necessarily have historically 
developed, and the comparative or empirical study of 
animal psychology clearly shows that this development 
can only be conceived of as a gradual evolution from the 
soul of vertebrate animals, as a gradual differentiation and 
perfecting which, in the course of many thousands of 
years, has led to the glorious triumph of the human mind 
over its lower animal ancestral stages. Here, as everywhere, 
the only way to arrive at a knowledge of natural truth is to 
compare kindred phenomena, and investigate their develop- 
ment. Hence we must above all, as we did in the examina- 
tion of the bodily development, compare the highest animal 
phenomena on the one hand with the lowest animal phe- 
nomena, and on the other with the lowest human phe- 
nomena. The final result of this comparison is this—that 
between the most highly developed animal souls, and the 
lowest developed human souls, there exists only a small 
quantitative, but no qualitative difference, and that this 
difference is much less than the difference between the 
lowest and the highest human souls, or than the difference 
between the highest and the lowest animal souls. 
In order to be convinced of this important result, it is 
above all things necessary to study and compare the mental 
