EVOLUTION IN BIOLOGY. 291 



belief of Aristotle whose writings he so often quotes, 

 and of whom he speaks as his precursor and model, 

 with the generous respect with which one genuine work- 

 er should regard another that such germs may arise by 

 a process of " equivocal generation " out of not-living 

 matter ; and the aphorism so commonly ascribed to him, 

 " omne vivum ex ovo" and which is indeed a fair sum- 

 mary of his reiterated assertions, though incessantly em- 

 ployed against the modern advocates of spontaneous 

 generation, can be honestly so used only by those who 

 have never read a score of pages of the " Exercitationes." 

 Harvey, in fact, believed as implicitly as Aristotle did 

 in the equivocal generation of the lower animals. But, 

 while the course of modern investigation has only brought 

 out into greater prominence the accuracy of Harvey's 

 conception of the nature and mode of development of 

 germs, it has as distinctly tended to disprove the occur- 

 rence of equivocal generation, or abiogenesis, in the pres- 

 ent course of nature. In the immense majority of both 

 plants and animals, it is certain that the germ is not 

 merely a body in which life is dormant or potential, but 

 that it is itself simply a detached portion of the sub- 

 stance of a pre-existing living body ; and the evidence 

 has yet to be adduced which will satisfy any cautious 

 reasoner that "omne vivum ex vivo" is not as well- 

 established a law of the existing course of nature as 

 "omne vivum ex ovo." 



In all instances which have yet been investigated, 

 the substance of this germ has a peculiar chemical com- 



