THE WONDERS OF LIFE 



and chemical experiments inspired a hope of attaining 

 similar results in embryology by means of the same 

 "exact" methods. But the application of them in this 

 science is only possible to a slight extent on account of 

 the great complexity of the historical processes and the 

 impossibility of "exactly" determining historical mat- 

 ters. This is true of both branches of evolution, in- 

 dividual and phyletic. Experiments on the origin of 

 species have very little value, as I said before; and this 

 is generally true of embryological experiments also. 

 However, the latter, especially careful experiments on 

 the first stages of ontogenesis, have yielded some in- 

 teresting results, particularly in regard to the physiology 

 and pathology of the embryo at the earliest stages of 

 development. The Archiv filr Entwickelungsmechanik, 

 which is edited by the chief representative of this school, 

 Wilhelm Roux, contains, besides these valuable inquiries, 

 a good number of ontogenetic articles, which partly rely 

 on and partly ignore the biogenetic law. 



Psychology and biogeny have been up to the present 

 regarded as the most difficult branches of biology for 

 monistic explanation, and the strongest supports of 

 dualistic vitalism. Both departments become accessible 

 to monism and a mechanico-causal explanation by means 

 of the biogenetic law. The close correlation which it 

 establishes between individual and phyletic development, 

 and which depends on the interaction of heredity and 

 adaptation, makes it possible to explain both. In regard 

 to the first, I formulated the following principle thirty 

 years ago in my first study of the gastraea theory: 

 " Phylogenesis is the mechanical cause of ontogenesis." 

 This single principle clearly expresses the essence of our 

 monistic conception of organic development: 



In the future every student will have to declare himself for or 

 against this principle, if in biogeny he is not content with a mere 

 admiration of the wonderful phenomena, but desires to under- 



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