212 EVOLUTION AND ANIMAL LIFE 



mechanical aids to observation it is also easy to see that the 

 smaller animals, the microscopic organisms, come into ex- 

 istence 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. 



The ancients believed that many animals were spontane- 

 ously generated. The early naturalists thought that flies 

 arose by spontaneous generation 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. 

 Aristotle, 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 Christian era 

 that these beliefs were attacked and began 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 produce the flesh flies develop from eggs IrJd 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 of a few kinds 

 of animals, like parasites and the animalcules of stagnant water. 

 It was maintained that parasites arose spontaneously from the 

 matter of the living animal in which they lay. Many parasites 

 have so complicated and extraordinary a life history that it was 

 only after long and careful study that the truth regarding their 

 origin was discovered. Bi*t in the case of every parasite whose 

 life history is known, the young are offspring of parents, of 

 other individuals of their kind. No case of spontaneous genera- 

 tion among parasites is known. 



