x.] BIOGENESIS AND A BIOGENESIS. 227 



day ; and, at the same time, by no means trustworthy. 

 But a magnifying power of 400 diameters, even when 

 definition reaches the exquisite perfection of our modern 

 achromatic lenses, hardly suffices for the mere discern- 

 ment of the smallest forms of life. A speck, only ~-th 

 of an inch in diameter, has, at ten inches from the eye, 

 the same apparent size as an object roo^th of an inch in 

 diameter, when magnified 400 times ; but forms of living 

 matter abound, the diameter of which is not more than 

 of an inch. A filtered infusion of hay, allowed 



to stand for two days, will swarm with living things, 

 among which, any which reaches the diameter of a human 

 red blood-corpuscle, or about g^th of an inch, is a giant. 

 It is only by bearing these facts in mind, that we can 

 deal fairly with the remarkable statements and specula- 

 tions put forward by BufFon and Needham in the middle 

 of the eighteenth century. 



When a portion of any animal or vegetable body is 

 infused in water, it gradually softens and disintegrates ; 

 and, as it does so, the water is found to swarm with 

 minute active creatures, the so-called Infusorial Animal- 

 cules, none of which can be seen, except by the aid of 

 the microscope ; while a large proportion belong to the 

 category of smallest things of which I have spoken, 

 and which must have looked like mere dots and lines 

 under the ordinary microscopes of the eighteenth 

 century. 



Led by various theoretical considerations which I 

 cannot now discuss, but which looked promising enough 

 in the lights of their time, Buff on and Needham doubted 

 the applicability of Kedi's hypothesis to the infusorial 

 animalcules, and Needham very properly endeavoured to 

 put the question to an experimental test. He said to 

 himself, If these infusorial animalcules come from germs, 

 their germs must exist either in the substance infused, or 



Q 2 



