300 The Outline of Science 



explorer was surely the Dutch observer Leeuwenhoek (1632- 

 1723), who discovered minute creatures like the Rotifers or 

 Wheel- Animalcules which are common in ponds, and the Infusori- 

 ans which abound wherever vegetable matter rots away in water. 

 He made numerous microscopes, and though they had neither 

 tube nor mirror, they were sufficient to enable him to demonstrate 

 his animalcules before the Royal Society of London, the Fellows 

 signing an affidavit that they had seen the little creatures. It was 

 Leeuwenhoek also who (in 1687) discovered bacteria, the very 

 minute organisms which cause all putrefaction, are responsible 

 for bringing about many diseases, and are yet of immense service 

 to many living creatures. 



It was not till long afterwards that Pasteur and others de- 

 monstrated the importance of bacteria, but it was a great event in 

 the history of science when Leeuwenhoek first proved their pres- 

 ence. It was literally the discovery of a new world with a teeming 

 population, with incalculable powers for good and evil. It must 

 have been a seed in the human mind, this idea of an intense activity 

 going on all unseen until men stuck lenses of glass in front of 

 their own. 



Another great event, though its importance was not recog- 

 nised till afterwards, was the discovery of the male elements or 

 spermatozoa of animals, which fertilise the egg-cells so that these 

 may begin to develop. This discovery was probably due (1677) 

 to a medical student in Leyden, Louis de Hamen, who showed 

 them to Leeuwenhoek, but it was not till more than a hundred 

 years later that the meaning of these sperm-cells was recognised. 

 And it is interesting to remember that it was not till 1843 that 

 another medical student, Martin Barry, in Edinburgh, observed 

 for the first time in the rabbit the fertilisation of the mammalian 

 ovum by the spermatozoon. In modern times an extraordinary 

 intensity of research has been focused on the usually microscopic 

 egg-cell and the always microscopic sperm-cell. In the union of 

 these an individual animal has its beginning and it is interesting 



