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, 

 Reaumur, 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 chef d'ceuvre of the opticians of that 



1 Needham, writing in 1750, says : 



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

 verit^ certaine, quetoute plante vient de sasemence spe'cifique, tout animal d'un 

 ceuf on de quelque chose d'analogue pr^existant dans la plante, ou dans 1'animal 

 de meine espece qui 1'a produit." Nouvelles Observations, p. 169. 



" Les naturalistes ontgeneralement cru que les animaux microscopiques etaient 

 engendre"s par des ceufs transportes dans Pair, ou de"pose"s dans des eaux dor- 

 mantes par des insectes volaus." Ibid. p. 176. 



