THE UNKNOWN FACTORS OF EVOLUTION. 99 



some unknown principle in Heredity ; if they are not trans- 

 mitted, there must be some unknown factor in Evolution. 



As regards Selection, we find more than the theoretical 

 objections advanced by Spencer and others. Neo-Darwinism 

 centers upon the principles of fortuitous variation, utility, and 

 selection as universal. In complete fossil series it is demon- 

 strated that these three principles, however important, are not 

 universal. Certain new adaptive structures arise gradually, 

 according to certain definite laws, and not by fortuity. 



Lamarck's and Buffon's factors afford at present only a par- 

 tial explanation of these definite phylogenic variations, even if 

 the transmission of acquired variations be granted. Nageli's 

 factor of certain constitutional lines of variation finds consid- 

 erable verification in fossil series as a principle of determinate 

 variation, but not as a general internal perfecting tendency. 

 St. Hilaire's factor of occasional saltatory evolution by sudden 

 modification of the hereditary mechanism is established, but 

 not as yet understood, although we are perhaps approaching 

 an explanation through experimental embryology. 



Our standpoint towards Variation in relation to all the Fac- 

 tors requires thorough reconsideration. The Darwinian law 

 of Fortuity and the Buffon law of the direct action of Environ- 

 ment, have hitherto been inductions from variations which may 

 be largely ontogenic and transitory. They both require con- 

 firmation on data of phylogenic variation. As for Lamarck's 

 factor, the evidence seems to be conclusive that somatogenic 

 variation is largely adaptive ; but it remains to be proved that 

 phylogenic variations as observed in human anatomy and in 

 palaeontology are invariably anticipated by corresponding 

 changes in the individual, in other words, that the definite 

 current of variation is guided by the inheritance of individual 

 reactions. 



Another consideration is, that individual Variation may play 

 a far less conspicuous r61e than we have assigned to it ; in 

 other words, that many of the most important changes in suc- 

 cessive generations are so gradual as to be entirely inconspicu- 

 ous in a single generation. 



Our conception of the mechanism or physical basis of 



