201 



VI. SUmilARY Al^ID CONCLUSIONS. 



During the lont; course of experi:nentation, the re- 

 nults of which are here set forth, the writer becffin? convinc- 

 ed that the deterioration in brain weight ^nd in the ability 

 to form habit9 were due, primarily, to the inbreeding. I v/ouTd 

 not iiaint?iin that inbreeding, per se, is necessarily produc- 

 tive of deleterious results if the parent stock be perfect 

 in every respect; but it is iripossible, by eny neans at our 

 comuand, to detei'inine physical perfection in any organism. 

 As only t'vo strains of inbred rats v/ere used it seems best 

 to ]et the question of the effects of inbreedin g remain open 

 until many strains of inbred rats, raised under different con- 

 ditions of nourishment, teinperature, etc., have been subject- 

 ed to parallel experiments. The two strains of rats used by 

 me v/ere the products of inbreeding, and their brains were ab- 

 solutely and relatively leas, on the averatje, in v/eight than 

 those of the normal rata used as control. And, too, the abili- 

 ty of the inbred rats to form habits agreed with the lesser 

 brain weight in that they were inferior in both these respects 

 to the normal control series. 



There vvere used in all the experi'ients one hundred 

 and t'';enty-f our rats: sixty-two inbreds and aixty-tv;o normal 

 controls. An equal number of males and females from inbreds 

 and normals were used in each experiment. Table XI shows the 

 distribution of relative brain weif^hts (with reference to body 

 lent^th) of the inbred rats and of the normal control series. 

 The inbred distribution is i-epresented by tlie lower curve, 

 that of the normal by the upper curve. The tireatest frequency 

 in the inbred curve occurs at .b6>: in the normal curve at 



