The Conditions of Life and Death. 95 



regarded the duckweed (Lemna), whose leaf-like shoots 

 are so common on the surface of pools, as a condensa- 

 tion of the still water, and a starting-point for higher 

 forms of plant life. 



While even Harvey continued to believe in sponta- 

 neous generation, the scientific attitude in relation to this 

 problem was at last represented by his Flor- R e di's EX- 

 entine contemporary, Francisco Redi (1626- periments. 

 97), distinguished alike as scholar, poet, physician, 

 and naturalist. By a few simple experiments he did 

 much to shatter the dogma of spontaneous generation, 

 and to establish the conclusion omne mvum e vivo. In 

 their own way these experiments are comparable to 

 those of Tyndall and Pasteur two hundred years later. 

 He showed that, if the flesh of a dead animal was pro- 

 tected with sufficient care from intruding insects, no 

 grubs or insects developed in it. It was, indeed, a 

 simple experiment, but no one had made it before! 

 Redi also tackled the problem of the origin of parasites, 

 but the cases he took were difficult, e.g. the maggots 

 inside a sheep's skull, and he did little beyond raising 

 the question. He was also baffled by the occurrence 

 of young insects within galls, and seems almost, in 

 spite of himself, to have been forced to conclude that 

 the galls produced the insects. 



We have already noticed that the origin of internal 

 parasites puzzled Aristotle, and it was long before any 

 solution was arrived at. To some it seemed enough to 

 suppose that they arose spontaneously from the juices 

 of their host; to others it seemed clearer to say that 

 they were created along with the host in the beginning, 

 and were handed on as part of the inheritance from 

 generation to generation. Thus Adam was said to 

 have contained all the human parasites from the first, 

 a state hardly consistent with Edenic bliss. The saga- 

 cious Leeuwenhoek (1632-1723) was one of the first to 

 insist that all the internal parasites of man and animals 

 came from outside, either as such or as germs, but he 

 did not prove his case. In fact, there was only one 

 way of proving it, namely, by experiment, but that was 

 not achieved until the nineteenth century, through the 



