14 



ZOOLOGY AS A SCIENCE 



cal worlds, biology is subjected to change. 

 This uncertainty that runs through all sci- 

 entific thinking is often very annoying to 

 people employed in pursuits which to them 

 possess more certainty. 



In order to illustrate the scientific method 

 in operation, an example is included here: 



1. Statement of the problem. Is vitamin 

 Bi necessary in the nutrition of a rat? 



2. Hypothesis. Since vitamin Bi is es- 

 sential in the nutrition of other mammals, 

 it is also essential in the nutrition of the rat. 



In order to solve this problem, it will be 



absence of growth, or decrease in growth 

 are used to measure effect. 



The rats are separated into two groups 

 of equal number, say five in each group. If 

 the sexes are evenly divided, the males and 

 females should be distributed evenly be- 

 tween the two groups. While it is unlikely 

 that sex differences affect the nutrition of 

 young rats, it is well to control all possible 

 factors. To the one group, known as the 

 experimental group, should be fed a care- 

 fully prepared diet that includes all food 

 essential to the growth of a rat, with the 



250- 



220 



^ I90 



c 



a 



_ 160 



5 I30-J 

 Si 



lOO- 

 10- 



controls 

 (ail weights dv/era< 



experimental 

 (a\\ weights averaged) 

 6i added 



30 35 40 46 50 55 60 



TIME IN DAYS 



65 



70 



75 



SO 



85 



Fig. 1-6. A graph showing the effect of the lack of vitamin Bi on the normal growth of rats. 

 The average weight of the control rats increased throughout the growth period, while the 

 rats on the experimental diet (lacking Bi) failed to grow after 5 days. When the vitamin 

 was restored to the diet they grew normally. 



necessary to obtain several rats, preferably 

 litter mates from a pure-bred stock, since 

 uniformity among the experimental animals 

 is most important. Litter mates are brothers 

 and sisters, therefore more alike than un- 

 related rats, and should give the same or 

 nearly the same response to treatment. It is 

 highly desirable that only one factor vary, 

 and that one should be the factor under 

 observation. From past experience it has 

 been found that young rats are more re- 

 sponsive to dietary differences than older 

 ones, and for that reason half-grown rats 

 should be used. The increase in growth, 



exception of vitamin Bi. To the other group, 

 the control group, should be fed the same 

 diet in equal proportions, but with vitamin 

 Bi added. 



3. Gathering facts from observations 

 and experimentation. The rats in both 

 groups must be weighed carefully at the 

 beginning of the experiment. Normal 

 growth charts for rats should be consulted 

 in order that growth in the control group 

 can be determined as normal. Each rat 

 must be weighed every day and accurate 

 records kept until the end of the experi- 

 ment. Food and water must be abundant at 



