RECENT ADVANCES IN SCIENCE. 563 



condition is that known as Heterotopia, in which fragments of tissues 

 or organs are found dwelling in a situation other than that which is 

 normal to them. This is particularly the case with certain glands, 

 such as the thyroid and suprarenal, but is also known with cartilage, 

 teeth, and the various constituents of dermoids. It no doubt occurs 

 by process of transplantation, the misplaced tissues developing no new 

 properties, but merely preserving their normal powers of growth. 

 The attempt to generalize from this fact and to attribute all tumor 

 formations to this cause carries the idea beyond its proper scientific 

 limits. 



Parasitism and Infection. 



With regard to the subject of parasitism, the progress of scientific 

 observation was retarded for centuries by the prevalence of the assump- 

 tion made by Paracelsus that disease in general was to be regarded as a 

 parasite. Pushed to its logical conclusion, this view would imply that 

 each independent living part of the organism would act as a parasite 

 relatively to the others. The true conception of a parasite implies its 

 harmfulness to its host. The larger animal parasites have been longest 

 known, but it is not so many years since their life history has been 

 completely ascertained and the nature of their cysts explained, while 

 an alternation of generations has been discovered in those which are 

 apparently sexless. Very much more recent is the detection of the 

 parasitic protozoa, by which the occurrence of the tropical fevers may 

 be explained. As yet we have not complete knowledge as to their life 

 history, but we hold the end of the chain by which this knowledge can 

 be attained. The elite of the infectious diseases are, however, the work 

 of the minutest kind of parasitic plants, bacteria, the scientific study 

 of which may be said to date from Pasteur's immortal researches upon 

 putrefaction and fermentation. The observation of microbes under 

 exact experimental conditions, and the chemical investigation of their 

 products opened up the modern field of bacteriology, a science among 

 the early triumphs of which were the discoveries of the bacilli of tuber- 

 cle and Asiatic cholera by Eobert Koch. In connection with this sub- 

 ject three important landmarks require comment. One is the necessity 

 for distinguishing between the cause and the essential nature of infec- 

 tious diseases, the latter of which is determined by the reaction of the 

 tissues and organs to microbes. Secondly, there is the relation between 

 the smaller parasites and the diseases determined by them. This may 

 be summed up in the general word [introduced by Professor Virchow 

 himself] "infection." But to assume that all infections result from 

 the action of bacteria is to go beyond the domain of present knowledge, 

 and probably to retard further progress. The third point is the ques- 

 tion as to the mode of action of infection. It is only the larger parasites 



