ATAVISM. 159 
Among reptiles, birds, and mammals, our knowledge at 
present only warrants us in regarding extra digits as 
reversions when they occur in non-pentadactyle animals. 
For instance, the spider monkeys have no thumbs; a 
careful dissection of the hand, however, will reveal, in 
connection with the trapezium, a band of fibrous tissue 
containing nodules of hyaline cartilage, representing the 
missing thumb. On one occasion I was surprised to 
find in an adult specimen of Adeles panniscus a small 
thumb projecting above the web of the finger and 
furnished with a nail. This condition is, | understand 
from competent zoologists, not infrequent, and as it is 
an example of the attainment of a functional condition 
by an organ usually suppressed in this species, it comes 
within the definition of atavism. Further, those mon- 
keys, its closest allies, marmosets, JZycetes, and Callithrix, 
possess functional thumbs. 
All cases of extra digits in non-pentadactyle mammals 
are not necessarily atavistic: let us consider this in 
reference to the horse. That the modern horse walks 
upon the enlarged third digit, and has a vestigial meta- 
carpal or metatarsal on each side of it, is accepted by 
morphologists. The comparatively recent ancestors of 
the horse had three functional digits. Hensel’s investi- 
gations on fHipparion mediterraneum indicate the pro- 
bability that the inner (second) digit was the last to abort. 
Horses are occasionally seen with two functional digits 
instead of one, and it is a noteworthy circumstance that 
in the majority of cases it is the inner digit which 
reappears, that is, the one which we should theoretically 
expect to reappear most frequently. Such cases are 
