Hdckel vs. Virch(nv. 297 



The evidence, upon which the theory of descent rests, shows 

 that the observed phenomena can only be explained by this 

 theory, and reduced to mechanical causes ; the guarantee of truth 

 lying in the fact that these ultimate causes are the same for all the 

 complicated processes of nature. He asks, where else can the 

 facts be found than among those of physiology and morphology, 

 paleontology and distribution, and the biological sciences in gen- 

 eral. We have also the evidence of actual experiments. Hackel 

 refers to the sponges, as yielding the most positive indication 

 that the term " species " has only relative value. He alludes to 

 the great progress of morphology, and finds in it justification for 

 his conclusions. He shows the importance of craniology, and 

 expresses the fear that Virchow and others have forgotten what 

 they wanted to prove by their measurements. 



That man has descended from the ape is nowhere assumed, but 

 from the order of apes — from one or more species now long extinct. 

 The theory does assert that the species in the long series of man's 

 immediate ancestors, were ape-like animals. Hackel states that the 

 relative certainty of this hypothesis cannot be compared, however, 

 with the absolute certainty of the general theory of descent ; 

 he considers all such hypotheses dependent upon the knowl- 

 edge existing at the time in which they are made. In the chapter 

 on " Cell Soul and Cellular Psychology," Hackel expresses his 

 opinion " that we must ascribe an independent soul-life to each 

 organic cell." He defines soul according to the two theories — 

 realistic and spiritualistic — adhering to the former, and adds that 

 Ehrenberg maintained that all Infusoria had nerves, mAiscles, and 

 organs of mind and sense. Hackel concludes that in the lowest 

 condition of the human ovum, the mental attributes of man begin 

 their course of development, and he finds in this a proof of his 

 realistic theory. 



Concerning methods of teaching, Hackel inquires where the 

 distinction can be made between subjective and objective know- 

 ledge ; he holds that all human knowledge is subjective, believ- 

 ing an objective science without theories inconceivable. He 

 reviews the several sciences, and finds that even in mathematics 

 the fundamental principles cannot be proved. The modern dis- 

 cussion as to a fourth dimension of space, is cited as an example 

 of the development of thought in the least expected direction. In 

 physics, what is known of the essence of matter, or of force ? 

 Gravitation is but an hypothesis. Electricity and magnetism, what 



