572 REPRODUCTION. 



egg, is continued through successive phases of growth, transformation 

 and maturity, and terminates at last with the production of an egg. 

 As this egg is similar to the first, the changes repeat themselves in their 

 previous order, and the indefinite continuance of the species is thus 

 established. 



Spontaneous Generation. The commonest observation shows that 

 the above facts hold good in regard to all animals and plants with 

 whose history we are fully acquainted. But it has sometimes been 

 surmised that there are exceptions to this rule ; and that living beings 

 may, under certain circumstances, be produced from inanimate mate- 

 rials ; thus presenting the singular phenomenon of a progeny without 

 parents. Such a production of organized bodies is known as sponta- 

 neous generation. Its existence is doubted by most physiologists, and 

 has never been positively established for any particular species ; but it 

 has been at various times the subject of discussion, forming a some- 

 what remarkable chapter in the history of physiology. 



It may be remarked in general that the organisms, supposed capable 

 of originating by spontaneous generation, have been always those 

 whose natural history was obscure, owing either to their, minute size 

 or to certain physiological peculiarities. Wherever animals or plants 

 appeared in abundance without evidence of the source from which they 

 came, it was formerly conjectured, for that reason, that their produc- 

 tion was spontaneous; and the ancient naturalists supposed all ani- 

 mals, except those which visibly lay eggs or produce living young, to 

 be formed by the fortuitous combination of their organic ingredients. 

 Maggots, shell fish, grubs, worms, and even some fishes w^ere thought 

 to be produced in this way, because they had no apparent specific origin. 



But further observation showed that these animals were really pro- 

 duced by generation from parents ; their secret methods of propa- 

 gation being discovered, and their relationship being detected by fol- 

 lowing the development of the young. A frequent obstacle to the 

 identification of species, in these investigations, is the interval which 

 elapses between the laying of the eggs and the subsequent appearance 

 of the young brood ; the new generation not showing itself until the 

 former has disappeared. A striking instance is that of the seventeen- 

 year locust (Cicada septendecim), where a period of seventeen years 

 intervenes between the hatching 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, most of these cases were gradually 

 traced to the usual 'method of generation from parents. 



Another source of error is the dissimilarity sometimes existing 

 between parents and young, especially when accompanied by a differ- 

 ence in their habits of life. Until the middle of the seventeenth century 

 there was no more undoubted instance of spontaneous generation than 

 the appearance of maggots in putrefying flesh. These creatures always 

 show themselves in meat at a certain stage of its decomposition ; they 



