THE UNKNOWN FACTORS OF EVOLUTION. 95 



tion ' an evolution principle which seems to be well supported 

 by palaeontological evidence. It is this : while the environ- 

 ment and the activity of the organism may supply the stimuli 

 in some manner unknown to us, definite tendencies of variation 

 spring from certain very remote ancestral causes ; for example, 

 in the middle Miocene the molar teeth of the horse and the 

 rhinoceros began to exhibit similar variations ; when these are 

 traced back to the embryonic and also to the ancestral stages 

 of tooth development of an early geological period, we discover 

 that the six cusps of the Eocene crown, repeated to-day in the 

 embryonic development of the jaw, were also the centers of 

 phylogenic variation ; these centers seem to have predetermined 

 at what points certain new structures would appear after these 

 two lines of ungulates had been separated by an immense 

 interval of time. In other words, upper Miocene variation was 

 conditioned by the structure of a lower Eocene ancestral type. 

 This is the proper place to recall a kindred conception of 

 Variation which has been in the minds of many, and has 

 been clearly formulated it appears by Waagen. It is of Varia- 

 tion so inconspicuous and so slight that it can only be recog- 

 nized as such when we place side by side two individuals 

 separated by a long series of generations. 1 Mark the contrast 

 with the extreme of St. Hilaire's saltatory evolution ; or again, 

 the contrast with Darwin's and Weismann's conception of 

 Variations, not, it is true, of a saltatory character, but as 

 sufficiently important and conspicuous to become factors in 

 the survival of the organism. This conception of ' phylogenic 

 variation,' as we have seen, is consistent with the application 

 of Galton's principles to human evolution, but it finds its 

 strongest support in palaeontology, and is the unconscious 

 motive of dissent on the part of all palaeontologists, so far 

 as I know their opinions, independently working in all parts of 

 the world, to the fortuitous Variation and Selection theory. 



1 This was brought out by the writer in his Oxford paper. See Nature, 

 August 30, 1894, p. 435. It has recently been independently stated with great 

 clearness by Scott in his article Variations and Mutations. American Journal of 

 Science, November, 1894. Scott, following Waagen, revives the term 'mutation' 

 for what Nageli has termed ' phylogenic variation.' 



