226 CRITIQUES AND ADDRESSES. [x. 



which passed through a totally different series of states 

 from those exhibited by the parent, and did not return 

 into the cycle of the parent ; this is what ought to be 

 called Heterogenesis, the offspring being altogether, and 

 permanently, unlike the parent. The term Heterogenesis, 

 however, has unfortunately been used in a different 

 sense, and M. Milne-Edwards has therefore substituted 

 for it Xenogenesis, which means the generation of 

 something foreign. After discussing Redi's hypothesis 

 of universal Biogenesis, then, I shall go on to ask how 

 far the growth of science justifies his other hypothesis 

 of Xenogenesis. 



The progress of the hypothesis of Biogenesis was 

 triumphant and unchecked for nearly a century. The 

 application of the microscope to anatomy in the hands of 

 Grew, Leeuwenhoek, Swammerdam, Lyonnet, Vallisnieri, 

 Re'aumur, and other illustrious investigators of nature of 

 that day, displayed such a complexity of organization in 

 the lowest and minutest forms, and everywhere revealed 

 such a prodigality of provision for their multiplication 

 by germs of one sort or another, that the hypothesis 

 of Abiogenesis began to appear not only untrue, but 

 absurd ; and, in the middle of the eighteenth century, 

 when Needham and Buffon took up the question, it was 

 almost universally discredited. 1 



But the skill of the microscope-makers of the eighteenth 

 century soon reached its limit. A microscope magnifying 

 400 diameters was a chefd'ceuvre of the opticians of that 



1 Needham, writing in 1750, says : 



" Les naturalistes modernes s'accordent unanimement a e"tablir, comme une 

 vrit6 certaine, quetoute plante vient de sasemence spe"cifique, tout animal d'un 

 oeuf on de quelque chose d' analogue preexistant dans la plante, ou dans 1'animal 

 dememe espece qui Fa produit." Nouvelles Observations, p. 169. 



"Les naturalistes ontgeneralement cru queles animaux microscopiques e"taient 

 engendr^s par des oeufs transports dans I'air, ou dposs dans des eaux dor- 

 mantes par des insectes volans." TfoW. p. 176. 



