PROBLEMS OF BIOLOGY 309 



his theory of auto-determination, and by the labours of the 

 school of Entwickelungsmechanik which he has founded, have 

 all in various ways, and from more or less different points 

 of view, helped to reconstruct and readjust our ideas of 

 the relations of embryological processes, and hence of the 

 phenomenon of Life itself, on the one hand to physical causes 

 (whether external to or latent in the mechanism of the cell), 

 or on the other to the ancient conception of a Vital Element, 

 alien to the province of the physicist. 



No small number of theories or hypotheses, that seemed 

 for a time to have been established on ground as firm as that 

 on which we tread, have been reopened in our day. The 

 adequacy of natural selection to explain the whole of organic 

 evolution has been assailed on many sides ; the old funda- 

 mental subject of embryological debate between the evolu- 

 tionists or preformationists (of the school of Malpighi, Haller, 

 and Bonnet), and the advocates of epigenesis (the followers of 

 Aristotle, of Harvey, of Caspar Fr. Wolff, and of Von Baer), is 

 now discussed again, in altered language, but as a pressing 

 question of the hour ; the very foundations of the cell-theory 

 have been scrutinised, to decide (for instance) whether the seg- 

 mented ovum, or even the complete organism, be a colony of 

 quasi-independent cells, or a living unit in which cell-differen- 

 tiation is little more than a superficial phenomenon ; the 

 whole meaning, bearing, and philosophy of evolution has 

 been discussed by Bergson, on a plane to which neither 

 Darwin nor Spencer ever attained ; and the hypothesis of 

 a Vital Principle, or vital element, that had lain in the 

 background for near a hundred years, has come into men's 

 mouths as a very real and urgent question, the greatest 

 question for the biologist of all. 



In all ages the mystery of organic form, the mystery of 

 growth and reproduction, the mystery of thought and con- 

 sciousness, the whole mystery of the complex phenomena 



