CXX PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May I909, 



' eternal,' but it is not ' rather worn out.' x The more intimate 

 and complete our knowledge of the facts, the more deeply are we 

 impressed with the imperfection of the record, and those who would 

 pronounce on the question of continuous or saltatory transformation 

 on such evidence as that just presented may be assumed to under- 

 estimate the duration of the unrecorded intervals in the genealogy. 

 The stages of evolution in the horse, as in other organisms, are 

 marked by changes in relative size of the different parts of the 

 skeleton, as well as by an increase in size of the whole. The latter, 

 or the phylogenetic growth, affords larger quantities for measure- 

 ment than the relative change of the parts, and is thus the more 

 convenient to consider in relation to time. The facts are excellently 

 summarized by Lull, 2 who gives the dimensions of the leading forms 

 of Equidse in the direct line of descent as follows : — 



EoMppus 11 inches, Wind Hirer Eocene. 



Protorohippus 14 ,, Wind River Eocene. 



Orokippus 16 ,, Bridger Eocene. 



Mesoldppus 18 ,, Oligocene White River. 



MioMppus 24 , , John Day, on the border-line between 



Oligocene and Miocene. 



Protohippns 36 ,, Loup Fork. 



PlioMp'pus 48 ,, Loup Fork, extends into Pliocene. 



Equus 64 ,, Pliocene to recent. 



Thus the total increase in height in passing from Eoliippus to 

 Equus is 64 — 11 = 53 inches ; that is about 10 inches per million 

 years. No doubt the course of evolution was not uniform, but it 

 was sufficiently so for the purpose of our illustration. If it pro- 

 ceeded continuously then each succeeding generation would be 

 taller than the immediately preceding by 0-00005 inch ; if by a 

 saltation, occurring on the average every 1000 generations, this 

 quantity would become 0-05 inch, but the smallest increase which 

 we could definitely ascertain by ordinary anatomical measurements 

 would be about 0*1 inch, and this would be brought about in 2000 

 generations or in a period, roughly speaking, of about 10,000 years. 



This example gives us some idea of the relative magnitudes with 

 which we have to deal ; it could be rendered much more effective 

 if it were not for a lamentable deficiency of the requisite data : we 

 look to time and the genius of our American colleagues to render 

 our quantitative knowledge in this direction more exact. 



No doubt, instances occur in which the rate of evolution among 



1 C. Deperet, ' The Transformations of the Animal World ' (authorized 

 English translation) London, 1909, p. 274. 



2 R. S. Lull, Am. Journ. Sci. ser. 4, vol. xxiii (1907) pp. 161-82. 



