708 RAYMOND PEARL. 



getting of food is of far more importance in tlie strnggle for 

 existence than the avoidance of danger. This point has, 

 however, been discussed earlier in the paper, and need not 

 detain us here. The real problem is presented in the attempt 

 to discover how any of the purposive reflex acts in the 

 organisms arose. I see no reason for denying that man}- of 

 them — such as, for example, the positive reaction which gets 

 the animal its food — were developed by natural selection. 

 There are other evidently purposeful reactions, however, with 

 whose development it hardly seems as if natural selection 

 could have had anything to do, since they cannot themselves 

 be of selective value. This point has been well brought out 

 in a recent paper by Morgan (: 02, p. 281). I think a pos- 

 sible explanation of some of these may be found in their 

 analysis into component factors, when it may appear that 

 only a very few simple reflexes had to be formed by natural 

 selection, and then all the reactions are built up from these. 

 An example will make my meaning clearer. In the righting 

 reaction of the planarian we have a fairly complex reaction 

 which is evidently immediately jDurposeful. Yet we find on 

 analysis that this reaction is at bottom nothing but a slight 

 modification of the ordinary negative reaction, which might 

 very well have been developed by natural selection. And 

 thus it is with other reactions and pieces of behaviour. They 

 are for the most part built up from a very few simple 

 purposive reflexes. If we can get them subdivided and 

 spread out, as it were, so that we can see what goes to 

 compose them, we may find that our problem has diminished 

 very much, and we shall have to deal with only a few factors 

 where before there appeared to be many. 



A difficult problem in purposeful behaviour presents itself 

 when we find that new methods of reaction appear at once if 

 the usual reaction is prevented. The best examples of this 

 are found in the righting reaction of cut pieces of planarians. 

 Here we find pieces of the body, in which the noannal 

 mechanism of the reaction has been destroyed, immediately 

 reaching a certain end (the righting) by a method differing 



