CHAPTER IX 
PRIMITIVE TYPES OF INTELLIGENCE IN 
CRUSTACEANS AND MOLLUSCS 
It is scarcely possible at present to fix, even with the 
rudest approximation, the point where intelligence makes 
its first appearance in the course of evolution. There is 
little doubt that the step from instinct to intelligence has 
been made, not once merely, but several times. The in- 
telligence of the higher Mollusca had, in all probability, an 
origin independent from that of the arthropods, and the in- 
telligence of the vertebrates was probably developed in- 
dependently of that of the other groups. Among the arthro- 
pods themselves it is not likely that the intelligence manifested 
by the arachnids had a common origin with that of the 
insects, and within both of these large groups intelligence 
may have been independently developed out of behavior of 
the purely instinctive type. 
Intelligence grows out of the complexity and perfection 
of the nervous mechanism, and along whatever line organiza- 
tion reaches a certain degree of development intelligence 
appears on the scene. From what has been said in previous 
pages we are prepared to appreciate the fact that intelligence 
is not an entirely new power unrelated to the other activities 
of organic life, but a process growing out of other organic 
functions and having the same end as these other functions; 
it is, as Spencer has so well emphasized, but a higher phase 
of those processes of adjustment and regulation which make 
up the life of the animal. 
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