August 24, 1906.] 



SCIENCE. 



229 



incipient and useless developments which 

 may suddenly, in their turn, be seized upon 

 by ever-watchful natural selection and 

 raised to a high pitch of growth and 

 function. 



The second, somewhat but by no means 

 altogether neglected, principle is that a 

 good deal of the important variation in 

 both plants and animals is not the variation 

 of a minute part or confined to one organ, 

 but has really an inner physiological basis, 

 and may be a variation of a whole organic 

 system or of a whole tissue expressing itself 

 at several points and in several shapes. In 

 fact, we should perhaps more generally 

 conceive of variation as not so much the 

 accomplishment and presentation of one 

 little mark or difference in weight, length 

 or color, as the expression of a tendency to 

 vary in a given tissue or organ in a par- 

 ticular way. Thus we are prepared for 

 the rapid extension and dominance of the 

 variation if once it is favored by selective 

 breeding. It seems to me that such cases 

 as the complete 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, favor this mode of con- 

 ceiving of variation. 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 re- 

 gions of the face. Of course, the alterna- 

 tive hairy or smooth condition of the in- 

 teguments both in plants and animals is a 

 familiar instance in which a tendency ex- 

 tending over a large area is recognized as 

 that which constitutes the variation. In 

 smooth or hairy varieties we do not pos- 

 tulate an individual development of hairs 

 subjected one by one to selection and sur- 

 vival or repression. 



The study of the physiology of un- 



healthy, injured or diseased organisms is 

 called pathology. It necessarily has an 

 immense area of observation and is of tran- 

 scending 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, pro- 

 ducing new and terrible forms of it by 

 their wilful interference with the earlier 

 rule of nature. 



Our knowledge of disease has been enor- 

 mously 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 natural- 

 ists, 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 Metchnikoff, the 

 zoologist and embryologist, that we owe the 

 doctrine of phagocytosis and the conse- 

 quent theory of immunity now so widely 

 accepted. 



We must not forget that in this same 

 period much of the immortal work of Pas- 

 teur on hydrophobia, of Behring and Roux 

 on diphtheria, and of Ehrlich and many 

 others, to whom the eternal gratitude of 

 mankind is due, has been going on. It is 

 only some fifteen years since Calmette 

 showed that if cobra poison were intro- 

 duced into the blood of a horse in less 

 quantity than would cause death, the horse 

 would tolerate with little disturbance after 

 ten days a full dose, and then day after 

 day an increasing dose, until the horse 

 without any inconvenience received an in- 

 jection of cobra poison large enough to kill 

 thirty horses of its size. Some of the 

 horse's blood being now withdrawn was 

 found to contain a very active antidote to 



