274 EVOLUTION AND THE PRESENT AGE 
ovum as exemplified in a thousand different species 
of animals, von Baer arrived at a group of technical 
formulas so general that they cover and describe with 
accuracy the series of changes that occur in all these 
cases. In other words, he made a general statement 
of the law of development for all physiological species. 
Now Spencer's great achievement was to prove that 
von Baer’s law of development, with sundry modifi- 
cations, applies to the succession of phenomena in the 
whole universe so far as known to us. 
Spencer took the development of the solar system 
according to the theories of Kant and Laplace, he took 
the geologic development of the earth according to the 
school of Lyell, he took the development of plant and 
animal life upon the earth’s surface according to Lin- 
nzeus and Cuvier, supplemented and rectified by Hooker 
and Huxley, and he showed that all these multifarious 
and apparently unrelated phenomena have through 
countless ages been proceeding according to the very 
law which expresses the development of dn individual 
embryo. In addition to this, Spencer furnished an 
especially elaborate illustration of his theory in a trea- 
tise upon psychology in which he traced the evolution 
of mind from the first appearance of rudimentary nerve 
systems in creatures as low as starfishes up to the most 
abstruse and complex operations of human intelli- 
gence, and he showed that throughout this vast region 
the phenomena conformed to his law. This was by 
far the profoundest special research that has ever been 
made on the subject of evolution, and it was published 
four years before Spencer had ever heard of Darwin’s 
theory of natural selection. 
In those days Spencer’s attitude toward such ques- 
