434 The American Naturalist. [May, 
when reaching a survival value will be acted upon by selection. 
I have recently” described as the ‘ potential of similar variation’ 
an evolution principle which seems to be well supported by 
paleontological evidence. It is this: while the environment 
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. 3 
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.” 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 
® Rise of the Mammalia in North America. Contr. Biol. Dept. Columbia Col- 
lege, vol. I, No. 2, September, 1893. 
” 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 terms ‘mutation’ for 
what Nägeli has termed ‘ phylogenic variation.’ 
