II THE PROGRESS OF SCIENCE 117 



have a totally different significance. Whether 

 not-living matter may pass, or ever has, under any 

 conditions, passed into living matter, without the 

 agency of pre-existing living matter, necessarily 

 remains an open question ; all that can be said is 

 that it does not undergo this metamorphosis under 

 any known conditions. Those who take a 

 monistic view of the physical world may fairly 

 hold abiogenesis as a pious opinion, supported by 

 analogy and defended by our ignorance. But, as 

 matters stand, it is equally justifiable to regard 

 the physical world as a sort of dual monarchy. 

 The kingdoms of living matter and of not-living 

 matter are under one system of laws, and there is 

 a perfect freedom of exchange and transit from 

 one to the other. But no claim to biological 

 nationality is valid except birth. 



In the department of anatomy and development, 

 a host of accurate and patient inquirers, aided by 

 novel methods of preparation, which enable the 

 anatomist to exhaust the details of visible structure 

 and to reproduce them with geometrical precision, 

 have investigated every important group of living 

 animals and plants, no less than the fossil relics of 

 former faunae and florae. An enormous addition 

 has thus been made to our knowledge, especially 

 of the lower forms of life, and it may be said that 

 morphology, however inexhaustible in detail, is 

 complete in its broad features. Classification, 

 which is merely a convenient summary expres- 



