DECEMBER 13, 1918] 
century students of animal behavior again re- 
turned to the problem which dominated be- 
haviorists of the early years of the seventeenth 
century, namely, the reduction of reactions to 
mechanical principles. Prominent among these 
students were Engelmann, Verworn, Loeb and 
Jennings. 
The work of the last three investigators 
mentioned is in a general way very well known. 
That of Engelmann, however, seems to have 
been to a considerable extent overlooked, al- 
though it is among the very best that has ever 
been done in behavior. I should like to refer 
particularly to his investigations on Huglena, 
published in Pfliiger’s Archiv in 1882, several 
years before any of the others mentioned be- 
gan work in this line. 
Engelmann finally concluded, after years of 
searching observations on the relation between 
physico-chemical phenomena and the reactions 
in various unicellular forms, that while many 
of the reactions in these forms are purely 
mechanical some of them can not be explained 
without postulating psychie processes. This 
conclusion may be responsible for the fact 
that his work has not received the attention 
that it deserves. 
Thus we see that one problem after another 
has dominated the work in behavior. Re- 
duction of reactions to mechanical principles; 
distribution of pain and pleasure; the evolu- 
tion of reactions and psychic phenomena; and 
again the reduction of reactions to mechanical 
principles. What has become of these prob- 
lems? What are the fundamental problems 
in behavior to-day ? 
DISTRIBUTION OF PLEASURE AND PAIN 
It has often been said that it is impossible 
to ascertain whether or not animals experience 
pleasure and pain and that it is consequently 
useless to attempt to ascertain the distribu- 
tion of such phenomena in the animal king- 
dom. In a sense this is true but in this sense 
it is also true in reference to human beings. 
Subjective states can be ascertained with 
certainty by the investigator only as they exist 
in himself. He can not be certain that your 
pain is like his pain. All that he can do is 
SCIENCE 
581 
to note his actions, including language, during 
the process of subjective experience, compare 
these actions with those in other individuals 
and base his conclusions upon the relation be- 
tween them. 
Precisely the same method is open to him in 
regard to other organisms, although it is evi- 
dent that comparison of actions becomes more 
and more difficult as the difference between 
the structure of the organisms involved in- 
creases. The problem as to the nature and ex- 
tent of pain and pleasure (feeling or sensa- 
tion) consequently becomes more and more 
difficult as one descends in the organic realm. 
This problem can, however, not be avoided. 
The behavior of every individual depends to 
a large extent upon his conclusion regarding 
the nature and extent of feelings in the 
creatures with which he comes in contact. 
Human society demands a decision of some 
sort or another regarding the distribution of 
these phenomena. Witness the work of the 
anti-vivisection organizations, societies for the 
prevention of cruelty to animals and charitable 
institutions everywhere, all built upon and 
acting upon decisions regarding this matter. 
The problem then resolves itself into this. 
Shall we permit human conduct in reference 
to such an important matter to rest upon judg- 
ments based upon evidence casually gained or 
shall we demand that it rest upon judgments 
based upon the results obtained in a compre- 
hensive comparative study of the reactions of 
organisms under experimentally controlled con- 
ditions ? 
Many anti-vivisectionists and members of 
other anti-organizations who shed copious 
tears over cats and dogs in our laboratories do 
not hesitate to sit all day and impale earth- 
worms, crabs and minnows on hooks, and they 
do not object to the practise in certain trop- 
ical regions of turning turtles and cutting 
steaks from them for a week or more while 
alive. They assume, of course, that earth- 
worms, crabs, fishes and turtles do not suffer. 
Are they correct in this assumption or was 
Brooks correct when, after a lifetime of in- 
timate association with animate beings of all 
sorts he said: “I try to treat all living things, 
