120 INTELLIGENT BEHAVIOUR 
therefore, is the establishment of a connection which is not 
provided through inheritance. To put the distinction in a 
brief form, we may say that instinct depends on how the 
nervous system is built through heredity ; while intelligence 
depends upon how the nervous system is developed through 
use. 
Assuming that an animal is capable of gaining experience 
and of acquiring new nervous connections in the course of 
individual experience, it follows that, as has already been 
indicated, instinctive behaviour in its logical purity is only 
presented in the first performance of any given co-ordinated 
act. For after this the animal has gained experience of its 
performance ; and this can no longer conform to a definition 
of instinct, according to which it is characterized as “ prior-to 
experience.” On the other hand, intelligent behaviour cannot 
be presented on the first occurrence of any action, since there 
is no prior experience thereof in the light of which it may be 
euided. This logical distinction may be expressed by saying 
that instinctive behaviour is always prior to experience, while 
‘ intelligent behaviour is always subsequent to experience. 
When, however, instinctive procedure continues throughout 
life practically unmodified or but little modified, we may still 
class it under instinct, since the hereditary connections are 
still the predominant factors. And where the latter part of 
an instinctive sequence is modified by the experience gained in 
the former part, we may still term the modification intelligent, 
however small may be the time-interval implied in the word 
“subsequent.” Sharp as the logical distinction is, the be- 
haviour of animals is in the main a joint-product, and whether 
we term it instinctive or intelligent depends upon whether the 
hereditary or the acquired factor predominates. 
Passing on now to consider some further characteristics of 
intelligent behaviour, we may first notice what Dr. Charles 
Mercier, in his work on ‘‘ The Nervous System and the Mind,” 
terms the four criteria of intelligence. Intelligence is mani- 
fested, he says, first in the novelty of the adjustments to 
external circumstances ; secondly, in the complexity ; thirdlv 
