56 VARIATION 



To fully appreciate the significance of this subject we need 

 to remind ourselves of evolutionary history with respect to digits. 



Man, for example, has normally five digits in all extremities. 

 The same is true of the bat. The ox has only two toes and the 

 horse but one, yet there are rudiments of others in both cases, 

 strongly suggesting that at some remote period the number might 

 have been greater. 



All things considered, it looks as if, for some unexplained and 

 at present unexplainable reason, animal life in most of its higher 

 forms had been originally constructed upon a plan of five as 

 regards the extremities. True, many, if not most species, have 

 long since departed more or less widely from the original plan, 

 and yet the numeral five is as distinctly characteristic of the digits 

 in animals as it is of petals in the rose family among plants. 



How this number has been gradually reduced to a final form, 

 sometimes of two, as in cattle, sometimes of one, as in horses, 

 is a chapter in development that belongs to the ancient history 

 of evolution. Moreover it is a chapter that, for obvious reasons, 

 must be read backwards and reconstructed from its fragments. 



It will assist in this reconstruction, and, what is of more 

 consequence to the breeder, it will throw much light upon the 

 manner of development and the unit of variability, if the stu- 

 dent will consider the present condition and evident ancestry of 

 a few characteristic species with respect to digits. 



Bats have five digits in both wing and leg, though the thumb 

 is modified into a strong claw. 



Birds have three digits in the wing, namely, i, the thumb, 

 which makes the so-called bastard wing; n and in, which make 

 the true wing ; iv and v, missing. Radius and ulna are both 

 present. In the leg the fibula is a mere splint, lying by the 

 tibia. 



There is but one metatarsus, but it is large and heavy, ending 

 in three pulley-like surfaces, over which play the tendons that are 

 attached to the three toes directed forward (n, in, and iv). This 

 plan suggests that the three middle metatarsals of the normal 

 foot have here become united along the shank into one, but 

 with three surfaces preserved for attachment of digits. Most 

 birds have also a toe behind. This is regarded as digit i, but 



