THE EVOLUTION OF LIFE 



representatives of the exact and descriptive sciences. 

 This shrinking from hypotheses often hides a defective 

 knowledge of other sciences, an incapacity for synthetic 

 thought, and a feeble sense of causality. The delusions 

 into which it leads many scientists may be seen from the 

 fact that chemistry, for instance, is reckoned an "exact" 

 science; yet no chemist has ever seen the atoms and 

 molecules of compounds with which he is occupied daily, 

 or the complicated relations on the assumption of which 

 the whole of modern structural chemistry is based. All 

 these hypotheses rest on inferences, not on direct obser- 

 vation. 



I have, from the first, insisted on the close causal 

 connection between ontogeny and phylogeny, ever since 

 I distinguished these two parts of biogeny in the fifth 

 book of the General Morphology. I also laid stress on 

 the mechanical character of these sciences, and en- 

 deavored to give a physiological explanation of their 

 morphological phenomena. Until then embryology had 

 been regarded as a purely descriptive science. Carl 

 Ernst Baer, who had provided a solid foundation for it 

 in his classic Animal Embryology (1828), was convinced 

 that all the phenomena of individual development might 

 be reduced to the laws of growth; but he was quite 

 unconscious of the real direction of this growth, its 

 "purposiveness," the real causes of construction. The 

 distinguished Wiirtzburg anatomist, Albert KoUiker, 

 whose Manual of Human Embryology (1859) gave the 

 first comprehensive treatment of the science from the 

 cellular point of view, adhered, even in the fourth 

 edition (1884), to the opinion that "the laws of the 

 development of the organism are still completely un- 

 known." In opposition to this generally received 

 opinion, I endeavored, in 1866, to prove that Darwin had, 

 by his improvement of the theory of descent, not only 

 solved the phylogenetic problem of the origin of species, 



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