THE GROWTH OF BIOLOGICAL IDEAS 145 



authoritative and not open to correction. It is not easy now- 

 adays to understand the spirit of those times, when biologists 

 were not expected to discover new facts, but only to expound 

 and illustrate the old opinions. Progress demanded not a 

 revival of the ancient knowledge but a breaking down of the 

 belief in the infallibility of the writers of antiquity. When 

 at last this tradition was broken, largely through the initiative 

 of the anatomist and physiologist Andreas Vesalius (page 77), 

 new knowledge of living organisms came rapidly; so rapidly, 

 indeed, that the old knowledge was soon of relatively small 

 importance, and it can scarcely be regarded as the basis of 

 modern biology. For this reason the biology of antiquity, 

 despite its considerable intrinsic interest, deserves only a 

 passing mention in a short history. 



Modern biology may be said to have originated about 1537, 

 when Vesalius left his native Belgium, settled in the Uni- 

 versity of Padua, and began to become influential. From 

 then onward progress has been more or less continuous. 

 Nevertheless, it is convenient to divide the history of mod- 

 ern biology into earlier and later periods; and 1838 is a con- 

 venient year from which to date the later period. The first 

 decades of the nineteenth century were a time of steady ad- 

 vance in several departments of biology. In 1838 this steady 

 advance was suddenly followed by spectacular discoveries. 

 The cell theory, enunciated by Schleiden in 1838, led to an 

 outburst of cytological research; and the study of the minute 

 structure of organisms received a second great stimulus from 

 the re-introduction of the staining technique about a decade 

 later. Then in the fifties came the first understanding: of 

 the alternation of generations in plants, and Dar^vin's and 

 Wallace's theory of evolution by natural selection. All these 

 advances, following one another in rapid succession, make it 

 reasonable to date the later period of modern biology from 

 the year 1838. Our history will therefore be related in tw^o 

 sections, the first covering the three centuries that started in 

 15^7, and the second dealing with the rapid advances that 



