3IO GREAT MEN OF SCIENCE 



it as thoroughly as possible. What goes beyond this 

 could never be quite forgotten by the more farseeing, in par- 

 ticular by physicians, though it might be called, as already by 

 Socrates and Plato, and then by Paracelsus, Descartes, and 

 later writers, some such thing as soul, spirit, archaeus, or 

 'life-force,' and though recognition of it by the learned 

 might pass through the greatest possible variations.^ The 

 investigation of the mechanism of living things made use of 

 all the means and knowledge already provided by scientific 

 research in general. 



In the course of two centuries, progress was made, from 

 Harvey to Wilhelm Weber, in the investigation of the 

 coarser mechanisms; the latter together with his brother, 

 investigated for the first time the blood wave which causes 

 the pulse, and must be distinguished from the flow of the 

 blood, and the mechanics of locomotion. In the case of the 

 finer organisms, the application of the optical investigations 

 of Galileo and Kepler ever-increasing improvements in lenses 

 and microscopes lead to starting from Toricelli's simple 

 glass ball, which was so long used with success; in this way 

 smaller animals also could be investigated as regards their 

 internal anatomy, and their manner of life and reproduction, 

 and the smallest forms of life were discovered. Already in 

 1650, Leeuwenhoek and Swammerdam^ proceeded to in- 

 vestigate the metamorphosis of insects, and to discover the 

 blood corpuscles and the infusoriae; and a hundred to two 

 hundred years later, the cellular nature of the structure of 

 all living matter was made clear, the cells and their contents 

 proving to be the essential carriers of life.^ 



1 The English reader may be referred to W. McDougall's Body and 

 Mind: A History and a Defense of Animism, London, 191 1. 



2 Leeuwenhoek Hved between 1632 and 1723 in Holland; he used only 

 simple lenses which he ground himself. Swammerdam lived between 

 1637 and 1680, likewise in Holland. 



' Here the very gradual progress made appears to have depended almost 

 entirely upon the gradual progress in perfection of the appliances and of 

 the use of the microscope. 



