THE MODERN STUDY OF ZOOLOGY 19 



resemble existing forms more closely than do those 

 of the older beds is simply that there has been a 

 continuous process of evolution ; that the fossils of 

 the recent beds are the ancestors of the now living 

 forms, the descendants of the fossils of the older 

 beds, and thus occupy an intermediate position 

 genealogically ; in which case their intermediate 

 position structurally becomes at once intelligible. 



Recognising these facts, attempts were soon 

 made to reconstruct the path of descent, to trace 

 out the pedigree of some given species. This was 

 first accomplished with any degree of success in 

 the case of the horse. The horse, the zebra, and 

 the ass stand alone among mammalia in pos- 

 sessing but one complete functional toe on each 

 limb ; they must either have been specially created 

 as such, or must have been derived from some 

 more typical mammalian form. Owing to their 

 large size, the fact that bones are fairly easily pre- 

 served as fossils, and the great time that must have 

 elapsed during the gradual transformation of some 

 typical mammal to the highly specialised horse, it 

 is only reasonable to expect that some direct evidence 

 of this transformation should be forthcoming. 

 Consequently this furnishes a very good test case. 

 Without entering into details, which would be un- 

 necessary in what is now so familiar an instance, 

 it will suffice to state that a series of fossil forms is 

 now known furnishing a complete gradation from 

 older tertiary forms with four or five toes on each 

 foot, through newer tertiary forms to the horse with 

 its one functional toe on each limb ; that, in other 



