Missing Links 
species, since these must have been spread over 
so many ages, and have existed in such numbers, 
that it seems impossible to account for their total 
absence from deposits in which great numbers of 
species belonging to other groups are preserved 
and have been discovered.” 
Wallace’s reply is to the effect that in the case 
of many species paleontology affords abundant 
evidence of the gradual change of one species into 
another, the foot of the horse being a well-known 
case. The genealogy of this noble quadruped 
can be traced from the Eocene four-toed Ovohzp- 
pus, through the Mesohippus, the Miohippus, the 
Protohippus, and the Plohippus, until we reach 
the one-toed Zguus. 
Wallace further points out that in order that 
the fossil of any organism may be preserved, the 
“concurrence of a number of favourable condi- 
tions” is required, and against this the chances 
are enormous. Lastly, he urges the imperfection 
of our knowledge of the things that lie embedded 
in the earth’s crust. 
The objection based on the lack of “ missing 
links” loses some of its force if we accept the 
theory that species sometimes arise as sports. 
Thus, suppose a species with well-developed 
horns produces as a mutation a hornless variety, 
which eventually replaces the horned form, we 
should look in vain for any forms intermediate 
between the parent and the daughter species. 
41 
