274 NATURAL HISTORY. 



Put briefly, the theory of progressive development is 

 that the primitive cells, which constituted on this 

 hypothesis the original forms of life, and which had 

 been presumably produced by spontaneous generation, 

 were advanced 'through a succession of higher grades 

 and a variety of modifications/ in obedience to some 

 law of an absolute nature, the whole process being 

 analogous to the embryonic development of an individual 

 animal. Just as each individual animal passes through 

 a series of changes during the course of its development 

 these changes taking place in a fixed order so the writer 

 of the ' Vestiges' supposes that the primordial forms of 

 life also passed through a series of developmental changes, 

 the different stages of their development being represented 

 by the life-assemblages of the successive great geological 

 periods. These developmental changes are supposed to 

 have taken place in a fixed order, and to have been 

 progressive in character; and the present forms of life 

 are supposed to represent the final term in the develop- 

 mental cycle of these hypothetical primordial cells. It 

 is not necessary to enter here into any discussion of 

 the theory of progressive development. The obvious 

 objection that an evolutionist of the Spencerian school 

 would take to it is that, from his point of view, 'the 

 impulse which has been imparted to the . forms of life/ 

 and to which their subsequent progressive development is 

 supposed to be due, is a mere metaphysical conception, 

 a hypothetical and scientifically inadmissible agency. 



