IN THE LAST HALF-CENTURY. 115 



Turning now to the great steps in that Bio- 



. logical 



vast progress which the biological sciences sciences. 

 have made since 1837, we are met, on the 

 threshold of our epoch, with perhaps the 

 greatest of all — namely, the promulgation 

 by Schwann, in 1839, of the generalisa- 

 tion known as the ' cell theory,' the appli- Th.o * ceil 

 cation and extension of which by a host of 

 subsequent investigators has revolution- 

 ised morphology, development, and physi- 

 ology. Thanks to the immense series of 

 labors thus inaugurated, the following fun- 

 damental truths have been established. 



All living bodies contain substances of Funda- 



mental 



closely similar physical and chemical com- truths 

 position, which constitute the physical lished. 

 basis of life, known as protoplasm. So 

 far as our present knowledge goes, this 

 takes its origin only from pre-existing 

 protoplasm. 



All complex living bodies consist, at 

 one period of their existence, of an aggre- 

 gate of minute portions of such substance, 



