244 Genetical Studies in Moths 



elements in the normal food, would in time be destined to replace the type 

 since we have demanded that the germ cells are altered in potentiality 

 and since we know that the germ plasm is continuous. 



Now have we any proof of the possible existence of such a scheme 

 of changed metabolism and of its not being disadvantageous to the 

 organism involved ? Not in the Insecta save by inference ; we have its 

 occurrence definitely^ in the case of human beings. 



In man occurs such a disturbance known as alcaptonuria. In this 

 condition the urine becomes very abnormal ; such bodies as tyrosin and 

 its allies instead of having their benzene nuclei completely disintegrated 

 by the slow combustion of vital processes, break down and yield inter- 

 mediate products like uroleucic and homogentisic acids which when 

 laid aside and submitted to hydrolysis or oxidation yield dark coloured 

 pigments. These bodies, respectively dihydroxy phenyl lactic acid and 

 dihydroxyphenyl acetic acid, are benzene derivatives possessing the 

 necessary hydroxyl groups qualifying them for being acted upon by 

 tyrosinase. 



A person with metabolism of this curious type cannot be regarded 

 as diseased ; he lives and enjoys life quite unhandicapped. Any natural 

 selection would fail to act against him ; nay, should ever food be presented 

 to him better dealt with by his than by the normal method then he 

 would be the one with a start in life's race. A very important feature 

 about this affection is that the available evidence^ directly states that 

 the condition is inherited. 



There is thus a very suggestive parallelism between this disturbance 

 and that formulated as the possible cause of melanism. 



Grant then that, in the haemolymph and in the excretory products 

 it contains after the reconstruction of the pupae succeeding histolysis, 

 we have greater supplies than the normal of compounds for the insect 

 tyrosinase to work on, then another fact favours the deposition of a 

 melanin in quantity. Experiment has shown that ferments such as 

 maltase have their action greatly accelerated by the presence of small 

 quantities of such salts as potassium and sodium chlorides, manganese 

 sulphate and so on. Have we not such present ? They, especially the 

 alkaline salts, must be taken in quantity on town grown foliage and 

 cannot be excreted normally. They can thus during pigment formation 

 affect the quantity produced and thus darken the insect. 



We shall now return to such cases as the melanism of Boarmia 



1 And locally in the melanosarcoma of horses, particularly of grey ones. 



2 Kirk, Brit. Med. Journ. 2, 1017 (1886) ; Journ. Anat. and Phys. 23, 69 (1889). 



