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 other 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 
Redi 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 zodlogists 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 
