14Q BIOLOGY AND ITS MAKERS 
But when the broad generalization, that all the tissues of 
animals and plants are composed of cells, was given to the 
world by Schleiden and Schwann, in 1838-39, the entire or- 
ganization of living forms took on a new aspect. This was 
progress in understanding the morphology of animals and 
plants. 
Protoplasm.-— With improved microscopes and attention 
directed to cells, it was not long before the discovery was 
made that the cells as units of structure contain protoplasm. 
That this substance is similar in plants and animals and is 
the seat of all vital activity was determined chiefly by the 
researches of Max Schultze, published in 1861. Thus step 
by step, from 1758, the date of the tenth edition of the 
Systema Nature, to 1861, there was a progress on the mor- 
phological side, passing from the organism as a whole to 
organs, to tissues, to cells, and finally to protoplasm, the study 
of which in all its phases is the chief pursuit of biologists. 
The physiological side had a parallel development. In 
the period of Linnzeus, the physiology of the organism was 
investigated by Haller and his school; following him the 
physiology of organs and tissues was advanced by J. Miiller, 
Bichat, and others. Later, Virchow investigated the physiol- 
ogy of cells, and Claude Bernard the chemical activities of 
protoplasm. 
This set forth in outline will be amplified in the follow- 
ing chapters. 
