34 Heredity. 



myopia, pigment-atrophy of the retina, polyuria (production of 

 excessive amounts of urine), predisposition to mental diseases, to 

 progressive muscular atrophy, to tumors, to obesity and to cer- 

 tain malformations (dwarfism, polydactylism, harelip, etc.). The 

 anomalous conditions mentioned are due, perhaps, to germinal 

 variations, possibly first brought into active manifestation by 

 the introduction of some external disturbing factor. (Vide Rib- 

 bert, Lehrbn'ch der all gem. Pathologie. 1901.) 



What parts respectively are taken by heredity and by external 

 influences in the development of congenital affections is in many 

 instances difficult or even impossible to determine, as our knowl- 

 edge of these matters is still deficient. This is particularly true 

 regarding the so-called hereditary defects of the domestic animals. 

 Among the diseases included in this category for the last century 

 or more, some are surely not inherited and not transmissible but 

 occur because of external influences ; for example, intermittent 

 ophthalmia, cataract, asthma, ringbone, frog-thrush. Others, as 

 dumb-staggers ["blind-staggers"], hsemoglobinsemia, curb, spavin, 

 deficient hoof-formation, may be caused directly by external in- 

 fluences, although it cannot be denied that inherited structural de- 

 fects may indirectly favor their appearance.* As instances of 

 direct inheritance and congenital origin may be mentioned many 

 cases of goitre in animals. 



*Cf. Dleckerhoff. Veher d. Erhfehler hei Zuchtpferden: Zcitschr. f. Vctcrinar- 

 hunde, 1902, Feb., p. 53. 



