XII] Heredity and Pathology 233 



example, albinism is almost certainly due to the absence 

 of at least one of the factors, probably a ferment, which is 

 needed to cause the excretion of pigment ; and, as Garrod 

 has shown, alkaptonuria must be regarded as due to the 

 absence of a certain ferment which has the power of de- 

 composing the substance alkapton. In the normal body 

 that substance is not present in the urine, because it has 

 been broken up by the responsible ferment ; but when the 

 organism is deficient in the power to produce that ferment, 

 then the alkapton is excreted undecomposed and the urine 

 is coloured by it. 



Similarly when we find that a condition such as retinitis 

 pigmentosa sometimes descends in one way and sometimes 

 in another, we may perhaps expect that a fuller knowledge 

 of the facts would show that more than one pathological 

 state may be included under the same name. 



It need scarcely be remarked that when a disease, such 

 as tuberculosis, which is due to a pathogenic organism, 

 affects certain families or strains with special frequency, the 

 hereditary or transmitted property is either the presence of 

 something which renders the organism specially liable, or 

 the absence of something which confers a higher degree 

 of resistance. From the nature of the case pedigrees are 

 not of much service in the analysis of these examples, for it 

 cannot be asserted that an individual who escapes under- 

 went the same risk of Infection as those who took the 

 disease. Though it cannot be doubted that the study of 

 the descent of disease-resistance is of the highest Import- 

 ance, both theoretical and practical, progress must here 

 be made by careful breeding experiments with animals and 

 plants which can be tested by uniform methods of infection. 

 As yet we know only one clear example in which such a 

 rule of descent has been ascertained, namely that discovered 

 by BIffen (29), who proved that. In wheat, resistance to 

 Rust-disease Is a recessive. In that classical example there- 

 fore we may suppose that this resistance Is due to the absence 

 of some ingredient which Is present in common wheats 

 and enables the rust-fungus to attack them successfully. 

 Varieties in several degrees possessing the property of re- 

 sisting disease are known in many orders of plants, and 

 though regarding animals less is known, there are some 



