52 Evolution and Adaptation 



the five-fingered forms are those from the older rocks, and 

 the three-toed forms from more recent layers. The value 

 of this kind of evidence might have been open to greater 

 doubt had the series been made up of forms found scattered 

 over the whole world, for it is well known how difficult it is 

 to compare in point of time the rocks of different continents. 

 But in certain parts of the world, especially in North America, 

 series of fossil horses have been found in sedimentary de- 

 posits that appear to be perfectly continuous. This series, 

 by itself, and without regard to the point as to whether in 

 other parts of the world other series may exist, shows exactly 

 those results which the theory of descent postulates, and we 

 find here, in all probability, a direct line of descent. While it 

 may be freely admitted that no such series can demonstrate 

 the theory of descent with absolute certainty, yet it would be 

 folly to disregard evidence as clear as this. 



In regard to the other point raised by Fleischmann 

 concerning the large number of species of fossil horses that 

 have existed in past times, it is obvious that while this greatly 

 increases the difficulty of the paleontologist it is not an 

 objection to the descent theory. In fact, our experience with 

 living species would lead us to expect that many types have 

 been represented at each geological period by a number of 

 related species that may have inhabited the same country. On 

 the descent theory, one species only in each geological period 

 could have been in the line of descent of the present species 

 of horse. The difficulty of determining which species (if 

 there were several living in a given epoch) is the ancestor 

 of the horse is increased, but this is not in itself an objection 

 to the theory. 



The descent of birds from flying reptiles is used by 

 Fleischmann as another point of attack on the transmuta- 

 tion theory. The theory postulates that the birds have come 

 from . ancestors whose fore-legs have been changed into 

 highly specialized wings. The long vertebrated tail of the 



