I. INTRODUCTION 



A few years ago experimental inbreeding of the albino rat, 

 Mus norvegicus albinus, was started at the Wistar Institute of 

 Anatomy and Biology in order to determine the anatomical 

 consequences of such procedure upon successive generations of 

 progeny. Among other results obtained was a distinct and 

 progressive decrease in actual and relative brain weight (relative, 

 that is, in reference to body length) for four generations of 

 close inbreeding. At the end of the fourth generation the rats 

 seemed lacking in vitality and, for this reason, were subjected 

 to a change in food. From this period until the end of the 

 tenth generation (the extent of inbreeding at the time this 

 paper was prepared) the relative brain weight remained, on 

 the average, constant at six and one-half per cent less than that 

 of the normal control rats. 



When, early in October, 1911, Dr. Donaldson suggested to 

 Professor Watson that the decrease in brain weight might be 

 accompanied by a similar decrease in ability to form habits 

 a new line of investigation in comparative psychology was 

 opened up. The problem was offered to the writer and gladly 

 accepted. 



It is no part of the purpose of this paper to raise the question 

 as to whether inbreeding, per se, results deleteriously upon the 

 progeny. In this, as in all disputed questions, it is unsafe to 

 be arbitrary, and authoritative testimony must await the results 

 of further investigations. We know, upon the authority of 

 historians, that the Incas of Peru for many generations married 

 their sisters and were physically and mentally superior to their 

 subjects. Breeders of domestic animals frequently resort to 

 inbreeding in order to perfect desirable qualities in the strain. 

 It may be, as many claim, that inbreeding results deleteriously 

 only in cases where an hereditary taint, occurring in the common 

 ancestor, is strengthened in the progeny of a consanguineous 

 union. Of the rats used in the experiments hereinafter described, 

 it is not postulated that the lesser ability to form habits is neces- 

 sarily due either to inbreeding or to the environmental factor of 



