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 
