GROWTH OF BIOLOGY IK THE NINETEENTH CENTURY. 467 



of science and philosophers of former centuries— Haller, Leibnitz, 

 Cuvier— were wrecked on the problem of development; how they stood 

 impotent before it with their methods of research. 



That every animal, man included, is at the l)eginning of his life tem- 

 porarily a single cell; that this cell multiplies by frequently repeated 

 divisions; that the cells arrange themselves into germinal layei-s from 

 which again the single organs take their origin, and that it is by 

 the association of the cell communities, as they multiply, after many 

 metamorphoses, that the perfect creature is formed, are facts of the 

 correctness of which everybody can easily convince himself. They 

 are secure, permanent acquisitions of science. 



With the second question, on the other hand, we pass to the sphere 

 of hypothesis. How did the organisms that are living to-day arise in 

 the course of the earth's history? Certainly an investigator well 

 schooled in philosophy will consider it to be a universal truth that the 

 organisms which to-day people the earth did not in bygone geological 

 ages exist in their present forms, but they, too, must have gone through 

 a process of development, beginning with the simplest forms, which 

 Haeckel has distinguished from the ontoganetical process by terming it 

 phylogenetical. The investigator will come to this conclusion by con- 

 necting different departments of biology. He will especially rely 

 upon the facts of individual development, which actually teach a 

 becoming of the complicated from the simpler. He will further 

 appeal to comparative anatomy, upon that philosophical science whose 

 erection has been brought in our century to high perfection by Cuvier 

 and Meckel, by Johannes Midler and Gegenbaur. 



But try to fully portray in detail in what special form a species 

 of animals of our day lived in the hoary antiquity, and the ground of 

 experience vanishes beneath you, for of the innumerable milliards of 

 creatures which lived in former geological periods — periods whose 

 duration is estimated in millions of years — only scanty remains of 

 skeletons have in exceptional cases been preserved in a fossil con- 

 dition. Of course from them we can gather but a very incomplete 

 and hypothetical idea of the soft bodies to which they once belonged. 

 Furthermore, it remains in every case undecided whether the descend- 

 ants of the ancient creature whose sparse remains we study did not 

 die out altogether, so that he can not be claimed as the ancestor of any 

 living form whatever. 



Twice in our century has the question of descent deeply stirred 

 both scientists and laymen and injected a powerful ferment into the 

 world of ideas. Brightly shine down upon us from history the oppo- 

 site names of Lamarck and Darwin. Lamarck, the great French 

 zoologist, wrote at the beginning of our century, at the tinie of the 

 German and French philosophy of nature, his famous Philosophic 

 Zoologique, a monument of freer philosophical consideration of the 



