NATURE OF REPRODUCTION. 671 



supposed that all species of animals, excepting those which visibly either 

 laid eggs or produced living young, were formed spontaneously from 

 the combination of their organic ingredients. Maggots, shell fish, grubs, 

 worms, and even some fishes were thought to be produced in this way, 

 simply because they had no apparent specific origin. 



But continued observation in natural history showed that in these 

 cases the animals were really produced by generation from parents ; 

 their secret methods of propagation being discovered, and their specific 

 identity being established by successive changes in development of the 

 young. The difficulty of doing this in any particular case is often in- 

 creased by the interval which elapses between the deposit of eggs by 

 the parents and the subsequent hatching of the young ; the new genera- 

 tion not showing itself until after the former has disappeared. A similar 

 instance is that of the American seventeen-year locust ( Cicada xepten- 

 decim), where a period of seventeen years intervenes between the hatch- 

 ing of the larva and the appearance of the perfect insect ; the larva all 

 this time remaining buried in the ground, while the life of the perfect 

 insect does not last over six weeks. But notwithstanding this difficulty, 

 all such doubtful cases were gradually traced to the usual method of 

 generation from parents. 



Another source of error was the great dissimilarity in the figure some- 

 times existing between the parents and their young, especially as this is 

 accompanied by an equal dissimilarity in their habits of life. Until 

 about the middle of the seventeenth century there was supposed to be 

 no more undoubted instance of spontaneous generation than the appear- 

 ance of maggots in putrefying meat. These creatures always show 

 themselves in meat at a certain stage of its decomposition ; they never 

 appear elsewhere ; and they do not themselves manifest the power of 

 producing young: and for these reasons they were believed to originate 

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

 But the simple experiments of Francisco Redi in 1668, demonstrated 

 the source of fallacy in this opinion and the true origin of the maggots. 

 He took, in the month of July, eight wide-mouthed glass bottles and 

 placed in them various pieces of dead flesh. Four of these bottles were 

 left open to the atmosphere, while the remaining four* were closed by 

 pieces of paper carefully adjusted over the mouth of each and fastened 

 by a cord round its neck. A short time afterward the flesh in the un- 

 covered bottles was filled with maggots, a peculiar kind of fly meanwhile 

 passing in and out by the open mouth ; but in the closed bottles not a 

 single maggot was visible, even after the lapse of 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 developed into perfect 

 insects similar to their parents. An extension of these observations to 

 other species of invertebrate animals made known a great variety of 

 instances in which the connection of parents and progeny might be traced 



