Tropisms and Instincts as Adaptations 405 
critically the facts, that we are not warranted in applying 
one explanation to those responses that are of use, and 
another to those that are of little or of no value. Inasmuch 
as the Darwinian theory fails to account for the origin of 
organs of little or of no value, it is doubtful if it is needed to 
explain the origin of the useful responses. If, on the other 
hand, we assume that the ovig7z of the responses has nothing 
to do with their value to the organism, we meet with no diffi- 
culty in those cases in which the response is of little or of 
no use to the organism. That great numbers of responses 
are of benefit to the organism that exhibits them can be 
accounted for on the grounds that those new species, that 
have appeared, that have useful responses, are more likely, 
in the long run, to survive, than are those that do not re- 
spond adaptively. 
We may now examine some of the more complicated 
responses and instincts, more especially those of the higher 
animals. Some of these are pure tropisms, z.e. definite re- 
sponses or reactions to an external exciting agent; others may 
be, in part, the result of individual experience, involving 
memory; others, combinations of the two; and still others 
may depend on a more complex reaction in the central ner- 
vous system of the animal. These cases can be best under- 
stood by means of.a few illustrations. 
As an example of a simple action may be cited a well- 
known reflex after cutting the nerve-cord of the frog, or 
after destroying the brain. If the frog is held up, and its 
side tickled, the leg is drawn up to rub the place touched. 
To accomplish this requires a beautifully adjusted system 
of movements, yet the act seems to be a direct reflex, involv- 
ing only the spinal cord. 
An example of a somewhat more complex reflex is the 
biting off of the navel-string by the mother in rodents 
and other mammals; an act eminently useful to the young 
animal, although of no importance to the mother herself. 
