The Wonders of Microscopy 301 



to trace this modern study, so important in connection with 

 heredity, back and back to the Leyden student's first glimpse of 

 spermatozoa. 



But we must not lose the wood in the trees: one of the real 

 wonders of microscopy, rising high above any mere curiosity 

 collecting, is the discovery of a world of invisible life. There are 

 the bacteria, which may be regarded as the simplest of living 

 creatures; there are the yeasts and the simple moulds; there are 

 the single-celled green plants which play so important a rdle in 

 the economy of the sea by providing food for humble animals like 

 water-fleas. There are the one-celled animals or Protozoa, such 

 as the chalk-forming Foraminifera, the Infusorians which often 

 serve as middlemen between the products of bacterial putrefaction 

 and some higher incarnation in crustacean or worm, and the death- 

 bringing organisms of malaria and sleeping-sickness. There are 

 also many-celled animals of miscroscopic dimensions, such as the 

 wheel-animalcules of the pond and the minute crustaceans which 

 play so important a part in the circulation of matter by feeding 

 on the microscopic Alga? and Infusorians in the water and being 

 themselves devoured by fishes. There are also the invisible early 

 stages of many important parasites, whose life-history would have 

 remained quite obscure if naturalists had been without micro- 

 scopes. It seems hardly too much to say that the system of 

 animate nature would be uncomfortably magical if the microscope 

 had not enabled us to detect the missing links in many a chain of 

 events. The liver-fluke which often destroys the farmer's sheep 

 is a relatively large animal about an inch long but it starts its 

 life as a microscopic egg which develops into a microscopic larva 

 that enters a water-snail, and has a remarkable history there. The 

 tapeworm, with which man becomes infected by eating bad beef 

 imperfectly cooked, may be several yards in length, but it began 

 as a microscopic egg which was swallowed by a bullock and 

 hatched into a microscopic boring larva, which eventually became 

 the beef-bladderworm. In hundreds of cases the microscope re- 



