iv PURSUIT OF KNOWLEDGE 73 



By his laborious studies in the latter half of the seven- 

 teenth century, Swammerdam worked out the complete 

 transformation of insects, and recognised the chief types 

 of development. For his facts he went direct to Nature, 

 and he was rarely deceived by her. Upon most of the 

 subjects studied by him, philosophers and other school- 

 men had been content to pass on fantastic ideas without 

 inquiry into their veracity. Aristotle, Vergil, Pliny and 

 other early writers, all agreed that certain bees, which 

 may sometimes be seen carrying small stones as they 

 fly, do so to prevent being blown out of their course in 

 windy weather. The conclusion was childish, but it 

 was sufficient for writers who had not watched the habits 

 of this insect the mason-bee. When actual observa- 

 tion was made by Swammerdam, nearly two thousand 

 years later, he found that the stones were used by the 

 bee to strengthen its hive. 



Aphids, or plant-lice, are familiar to every gardener, 

 yet how few know anything of their life-history or of 

 the patient work of investigators who revealed it. When, 

 toward the end of the seventeenth century, the ever- 

 curious naturalist, Antony Van Leeuwenhoek, began to 

 study the insects, he sought for their eggs, but without 

 success. Later, he made the surprising discovery that 

 aphids brought forth their young alive, and upon opening 

 an aphis only a fortnight old he found no fewer than 

 sixty young ones in it. Reaumur extended Leeuwen- 

 hoek's observations, and showed that both the winged 

 and wingless aphids could produce living young. He 

 tried to isolate aphids from birth to see if they would 

 still continue to increase their kind, but was prevented 

 by accidents from concluding his observations. When, 

 therefore, Charles Bonnet (1720-1793) asked him to 

 suggest a subject of investigation, the unfinished 



