THE MULTIPLICATION OF ANIMALS AND SEX 51 



spring of parents of their kind. And in these latter days of 

 microscopes and mechanical aids to observation it is even 

 easy to see that the smaller animals, the microscopic organ- 

 isms, come into existence only as they are produced by the 

 division of oijier similar animals, which we may call their 

 parents. But in the days of the earlier naturalists the 

 life of the microscopic organisms, and even that of many 

 of the larger but unfamiliar animals, was shrouded in 

 mystery. And what seem to us ridiculous beliefs were 

 held regarding the origin of new individuals. 



29. Spontaneous generation. The ancients believed that 

 many animals were spontaneously generated. The early 

 naturalists thought that flies arose by spontaneous genera- 

 tion from the decaying matter of dead animals ; from a 

 dead horse come myriads of maggots which change into 

 flesh flies. Frogs and many insects were thought to be 

 generated spontaneously from mud. Eels were thought to 

 arise from the slime rubbed from the skin of fishes. Aris- 

 totle, the Greek philosopher, who was the greatest of the 

 ancient naturalists, expresses these beliefs in his books. It 

 was not until the middle of the seventeenth century- 

 Aristotle lived three hundred and fifty years before the 

 birth of Christ that these beliefs were attacked and be- 

 gan to be given up. In the beginning of the seventeenth 

 century William Harvey, an English naturalist, declared 

 that every animal comes from an egg, but he said that the 

 egg might " proceed from parents or arise spontaneously or 

 out of putrefaction." In the middle of the same century 

 Eedi proved that the maggots in decaying meat which pro- 

 duce the flesh flies develop from eggs laid on the meat by 

 flies of the same kind. Other zoologists of this time were 

 active in investigating the origin of new individuals. And 

 all their discoveries tended to weaken the belief in the 

 theory of spontaneous generation. 



Finally, the adherents of this theory were forced to 

 restrict their belief in spontaneous generation to the case 



