239 



Muller \). The same cerebral regression by domestication was found 

 by Lapicque ') for the Ox and the Sheep, by Klatt *) and Betftcke *) 

 for the Ferret, by Lapicque *) and Timmann ') foj- tlie domestic Duck, 

 and now by me for the Domestic Dog. For 72 of Donaldson and 

 Hatai's wild Mus norvegicus ') of both sexes, of 335 to 525 gr., 

 averagely 389.861 gr. body weight, with averagely 2.402 gr. brain 

 weight, and 71 male and female wild rats of 275 to 325 gr., 

 averagely 300.211 gr. body weight, with an average brain weight 

 of 2.299 gr., I calculate an exponent of relation of 0.1674. Tiiat 

 this exponent is considerably smaller than is usually found between 

 individuals of one species, may be readily explained in this way 

 that Donaldson and Hatai give the body weights irrespective of 

 the state of adolescence and the fat percentage (of which they state 

 that it augments with age); part of the increase of the body weight 

 is, therefore, not accompanied by increase of the brain weight, as 

 is the case on comparison of adult individuals only, and which are 

 in a medium condition. 



In Fig. 4, after Donaldson ') the exponent of the individuals with 

 body weights between 250 and 446 gr. may be calculated at 0.1572 

 for the male wild Mus norvegicus (from observations of weights on 

 232 male specimens of all ages). From Donaldson's Table 85*) 

 the exponent 0.1554 may be calculated for body weight of 301.0 

 to 389.7 gr. The exponent is 0.1342 for the male albino of this 

 species of 181 to 350 gr. body weight. The relatively smaller increase 

 of the brain weight with increasing body weight of the (domestic) 

 albino Rat finds expression in the slower ascent of the curve and 

 the lower value of the exponent. It may be admitted that the 

 exponent is in general somewhat lower in the domesticated species 

 (not leading a natural life), because the brain increases somewhat 



') E. Muller, Vergleichende Untersuchungen an Haus- und Wildkaninchen. 

 Loc. cit. p. 503—588. 



') See note 9 foregoing page. 



*) See note 10 foregoing page. 



*) H. Bethcke, Vergleichende Untersuchungen an Frettchen und Iltissen. Ibid., 

 p. 589—620. 



6) 0. Timmann, Vergleichende Untersuchungen an Haus- und Wildenten. Ibid., 

 p. 621—656. 



«) Donaldson and Hatai, I.e., p. 426 — 427. 



7) From Chart 3], p. 201 in H. H. Donalsdon, The Rat. Reference Tables 

 and Data for the Albino Rat (Mus norvegicus albinus) and the Norway Rat (Mus 

 norvegicus). Memoirs of the Wistar Institute of Anatomy and Biology. N^. 6, 

 Philadelphia 1915. 



8) Ibid, p. 208. 



