154 
THE GEOLOGICAL MIDDLE AGE. 
tails, to balance the body after leaping. In these 
tracks we find traces of a tail between the feet. 
I do not bring this forward as any evidence that 
these animals were allied to Kangaroos, since I 
believe that nothing is more injurious in science 
than assumptions which do not rest on a broad 
basis of facts ; but I wisli only to show that these 
tracks recall other animals besides Birds, with 
which they have been universally associated. 
And seeing, as we do, that so many of the early 
types prophesy future forms, it seems not improb- 
able that they may have belonged to animals 
which combined with reptilian characters some 
birdlike features, and also some features of the 
earliest and lowest group of Mammalia, the Mar- 
supials. To sum up my opinion respecting these 
footmarks, I believe that they were made by ani- 
mals of a prophetic type, belonging to the class 
of Reptiles, and exhibiting many synthetic char- 
acters. 
The more closely we study past creations, the 
more impressive and significant do the synthetic 
types, presenting features of the higher classes 
under the guise of the lower ones, become. They 
hold the promise of the future. As the opening 
overture of an opera contains all the musical ele- 
ments to be therein developed, so this living pre- 
'ude of the Creative work comprises all the or- 
ganic elements to be successively developed in 
