THE NATURE OF REPRODUCTION. 573 



are never seen elsewhere; and they do not apparently possess the 

 power of producing young. For these reasons they were believed to 

 originate from the dead flesh and to die without leaving a progeny. 

 But the experiments of Redi in 1668, demonstrated the fallacy of this 

 opinion and the true urig-in of the maggots. He took, in the month 

 of July, eight wide-mouthed bottles and placed in them pieces of flesh. 

 Four of the bottles were left open to the atmosphere, while the remain- 

 ing four were closed by paper fastened over their orifices. In a short 

 time the flesh in the uncovered bottles was filled with maggots, a pecu- 

 liar kind of fly meanwhile passing in and out by the open mouth ; but 

 in the closed bottles not a maggot \7cd visible, even after several 

 months. 



Thus it was evident that the maggots were not formed from the dead 

 flesh, but that their germs came in some way from without ; and con- 

 tinued observation showed that they were hatched from eggs deposited 

 by the flies, and that after a time they became perfect insects similar 

 to their parents. An extension of these observations to other inver- 

 tebrate animals made known a great variety of instances in which the 

 connection of parents and progeny might be traced through several 

 intermediate conditions ; so that apparent differences in their configura- 

 tion and structure no longer offered a serious difficulty to the investi- 

 gator. As a general rule, since tls.it time, whenever a rare or compara- 

 tively unknown animal or plant has been suspected to originate by 

 spontaneous generation, it has only been necessary to examine thor- 

 oughly its habits and functions > to discover its real methods of propa- 

 gation, and to show that they correspond, in all essential particulars, 

 with the ordinary laws of reproduction. The limits within which the 

 doctrine of spontaneous generation could be applied have been thus 

 gradually narrowed, in the same degree that the study of natural his- 

 tory has advanced ; the presumption of its existence always hanging 

 upon the outskirts of definite knowledge, and relating only to those 

 animals or plants which are for the time imperfectly understood. The 

 two groups from which it has been most recently excluded are the 

 Entozoa and the Infusoria. 



I. Entozoa. These are parasitic organisms inhabiting the bodies of 

 other living animals, from whose organic juices they derive their nour- 

 ishment. 



There are many kinds of entozoa, all of which are confined, more or 

 less strictly, to certain parts of the body which they inhabit. They 

 are found in the intestines, the liver, the kidneys, the lungs, the heart, 

 and the blood-vessels ; some on the surface of the brain ; others in the 

 muscles or in the eyeball. Each parasite, as a rule, is peculiar to 

 the species of animal which it inhabits, and to a particular part of the 

 body, or even to a part of one organ. Thus, Ascaris lumbricoides 

 is found in the small intestine, Oxyuris vermicularis in the rectum, 

 Trichocephalus dispar in the caBcum. One kind of Distoma has its 



