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 origin 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, i.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. 



