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SCIENCE. 



[N. S. Vol. XXII. No. 555. 



of the scientific world on account of their 

 improbability. As an excellent instance 

 of these complicated relations may be cited 

 the life history of the European hook 

 worm, published by Looss, little more than 

 a year ago. Looss has followed the migra- 

 tion step by step from the time the minute 

 larvae penetrate the hair follicles of the 

 skin, enter a lymph space or a capillary to 

 be carried by the current through the ves- 

 sels ultimately into the right heart and 

 from it into the lungs, where they desert 

 the vascular system and migrate into the 

 air cells, and then, wandering outward 

 along bronchioles, bronchi and trachea, pass 

 over the dorsal margin of the larynx and 

 into the oesophagus, from which their path- 

 way lies directly back through the alimen- 

 tary canal to their final location in the 

 small intestine. The migration requires 

 ten weeks, during which time they pass 

 through molts and grow in size, attaining 

 the adult form and sexual maturity only 

 after arrival at the end of the journey. 

 Here the entire life cycle is passed in a 

 single host, but its different phases are 

 associated with various organs. In still 

 other cases among the Nematoda a free- 

 living generation alternates with the para- 

 sitic generation, instead of two kinds of 

 parasitic generations which are found in 

 different hosts. 



Concerning conditions among the Proto- 

 zoa there is less definite knowledge of the 

 life history than among the higher groups, 

 but instances of all the conditions cited for 

 the worms may also be found here. Some 

 species undergo direct development, others 

 make a single or even a double change of 

 hosts, and in some two generations of dif- 

 ferent type alternate in the complete life 

 cycle of the organism. Thus the amoeba 

 of tropical dysentery {Entamoeba histoly- 

 tica) seems to develop directly; the blood 

 amoeba of malaria {Plasmodium malarice) 



goes through an asexual reproductive cycle 

 in man and another, the sexual cycle, in 

 the mosquito. In this case we know that 

 the mosquito is not the mere mechanical 

 carrier of the disease germ, but that it is a 

 necessary link in the life history, a breeder 

 as well as a transmitter of disease. Re- 

 garding the role of the cattle tick in Texas 

 fever, it may be inferred with great proba- 

 bility that it plays a similar part, even 

 though the history of the parasite within 

 the tick has not yet been worked out. In 

 other diseases, such as sleeping sickness, 

 where the parasite, a flagellate protozoon, 

 known as a trypanosome, is transmitted by 

 a biting fly, familiarly called the tse-tse 

 fly, there is less evidence on which to base 

 a conclusion. The tse-tse fly may be purely 

 mechanical in its intervention; it seems 

 more probable, however, that it plays a 

 more intimate part. The instance shows 

 very clearly, however, that until the life 

 history has been elucidated, it is impossible 

 to determine the relative importance of any 

 element in the series, or intelligently to 

 combat the disease which evidently should 

 be attacked at its weakest point. This 

 factor will be considered more in detail 

 later on. 



But animals also stand in a causal rela- 

 tion to disease ; certain forms are definitely 

 shown to be producers of disease and in 

 this aspect demand especial consideration. 

 This fact has been generally recognized in 

 the case of a few parasites from the earliest 

 days of medical history. The fiery serpent 

 of the wilderness was no doubt the guinea 

 worm, of which the most ancient medical 

 writings make note ; and in this instance 

 not only the cause of the malady, but also 

 the general mode of infection through 

 drinking water, and the method of cure, 

 the removal of the worm, were known to 

 the Egyptian as well as to the Greek physi- 

 cians. But such instances are rare. Re- 



