18 A Century of Science 



worked up from the study of the tissues and their 

 properties. The path thus broken by Bichat led 

 to the cell doctrine of Schleiden and Schwann, ma- 

 tured about 1840, which remains, with some modi- 

 fications, the basis of modern biology. The ad- 

 vance along these lines contributed signally to the 

 advancement of embryology, which reached a start- 

 ling height in 1829 with the publication of Baer's 

 memorable treatise, in which the development of 

 an ovum is shown to consist in a change from ho- 

 mogeneity to heterogeneity through successive dif- 

 ferentiations. But while Baer thus arrived at the 

 very threshold of the law of evolution, he was not 

 in the true sense an evolutionist ; he had nothing 

 to say to phylogenetic evolution, or the derivation of 

 the higher forms of life from lower forms through 

 physical descent with modifications. Just so with 

 Cuvier. When he effected his grand classification, 

 he prepared the way most thoroughly for a general 

 theory of evolution, but he always resisted any 

 such inference from his work. He was building 

 better than he knew. 



The hesitancy of such men as Cuvier and Baer 

 was no doubt due partly to the apparent absence 

 of any true cause for physical modifications in spe- 

 cies, partly to the completeness with which their 

 own great work absorbed their minds. Often in 



