THE ADVANCE OF SCIENCE 133 



plete disappearance of scales from the integument of 

 some osseous fishes, or the possible retention of three 

 or four scales out of some hundreds present in nearly 

 allied forms, favour this mode of conceiving of varia- 

 tion. So also does the marked tendency to produce 

 membranous expansions of the integument in the bats, 

 not only between the digits and from the axilla, but 

 from the ears and different regions of the face. Of 

 course, the alternative hairy or smooth condition of the 

 integuments both in plants and animals is a familiar 

 instance in which a tendency extending over a large 

 area is recognised as that which constitutes the variation. 

 In smooth or hairy varieties we do not postulate an 

 individual development of hairs subjected one by one to 

 selection and survival or repression. 



Disease. The study of the physiology of unhealthy, 

 injured, or diseased organisms is called pathology. It 

 necessarily has an immense area of observation and is 

 of transcending interest to mankind who do not accept 

 their diseases unresistingly and die as animals do, so 

 purifying their race, but incessantly combat and fight 

 disease, producing new and terrible forms of it, 

 by their wilful interference with the earlier rule of 

 Nature. 



Our knowledge of disease has been enormously 

 advanced in the last quarter of a century, and in an 

 important degree our power of arresting it, by two 

 great lines of study going on side by side and originated, 

 not by medical men nor physiologists in the narrow 

 technical sense, but by naturalists, a botanist, and a 

 zoologist. Ferdinand Cohn, Professor of Botany in 

 Breslau, by his own researches and by personal train- 

 ing in his laboratory, gave to Robert Koch the start on 

 his distinguished career as a bacteriologist. It is to 



