II THE PROGRESS OF SCIENCE 115 



animals and plants. The intimate structure, and 

 the modes of change, in the cells of the two are 

 fundamentally the same. Moreover, the higher 

 forms are evolved from lower, in the course of their 

 development, by analogous processes of differen- 

 tiation, coalescence, and reduction in both the 

 vegetable and the animal worlds. 



At the present time, the cell theory, in 

 consequence of recent investigations into the 

 structure and metamorphosis of the " nucleus," is 

 undergoing a new development of great signi- 

 ficance, which among other things, foreshadows 

 the possibility of the establishment of a phy- 

 sical theory of heredity, on a safer foundation 

 than those which Buffon and Darwin have 

 devised. 



The popular belief in abiogenesis, or the so- 

 called " spontaneous " generation of the lower forms 

 of life, which was accepted by all the philosophers 

 of antiquity, held its ground down to the middle 

 of the seventeenth century. Notwithstanding the 

 frequent citation of the phrase, wrongfully 

 attributed to Harvey, " Omne vivum ex ovo," that 

 great physiologist believed in spontaneous 

 generation as firmly as Aristotle did. And it was 

 only in the latter part of the seventeenth century, 

 that Redi, by simple and well-devised experiments, 

 demonstrated that, in a great number of cases of 

 supposed spontaneous generation, the animals 

 which made their appearance owed their origin to 



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