The Presidenfs Address. By H. G. Plimmer. 133 



could not be found when the testes were not active. It is curious 

 that so keen an observer as Erasmus Darwin couki say of tlieni in 

 1794, ''perhaps they may be the creatures of stagnation or putri- 

 dity, or perhaps no creatures at all." But Leeuwenhoek stuck to 

 his views of their function, and to his opposition to the theory of 

 spontaneous generation, which had to wait almost until our time 

 before it was finally crushed by Tyndall and Pasteur. He fought 

 steadily against the view that living things are " bred from corrup- 

 tion," and showed that weevils (supposed to be bred from wheat as 

 well as in it) are grubs hatched from eggs deposited by insects : and 

 also that the sea mu?sel was not generated from sand and mud, 

 as Aristotle thought, but from spawn ; and he maintained that 

 the same was true of the fresh-water mussel, in whose ova he 

 described the rotation of the embryo, which is generally supposed 

 to have been discovered much later. He showed that eels were 

 not produced from dew, as was then supposed by " respectable and 

 learned men," as he calls the wise ones of his time ; and he sus- 

 tained generally the Harveian idea that all animals proceed from 

 ova, and he thus founded embryology. He says : " And many 

 with good reason judge that Nature keeps the same method in in- 

 visible creatures, that it does in all the sizes of visible, and that 

 even the least as well as the greatest, is made of seed, and that 

 one of the least creatures, as a horse-fly, can be no more made out 

 of corruption, than one of the greatest, as a horse." 



He glorified even the common flea — so carefully collected for 

 him by his little maidservant — and made the first exact observa- 

 tions upon this enemy of man. He described its structure and 

 traced out the whole history of the metamorphoses of " this minute 

 and despised creature," which some asserted to have been produced 

 from sand, dust, dung of pigeons, or urine ; and he showed that it 

 was " endowed with as great perfection in its kind as any large 

 animal." He also found that its pupa was attacked by a mite, the 

 knowledge of which fact gave rise later to the well-known lines 

 of Swift : — 



" So, naturalists observe, a flea 

 Has smaller fleas that on him prey ; 

 And these have smaller still to bite 'em, 

 And so proceed ad infinitum.'" 



His work on the eye deserves more than a mere mention. He 

 gave us the first true account of the structure of the optic nerve, 

 and showed that it was composed of filaments of nerve and " trans- 

 parent winding streaks " of connective tissue, and that it had no 

 tube down its centre, through which the light could get to the 

 brain, as was thought. 



He described the lens as being made of " orbicular scaly 

 parts," and figured it with accuracy sufficient for use to-day. He 

 says he kept some lens specimens to show, "although I have 

 already found more than once, that some have made their particular 



