34 EVOLUTION 
possibly ancestral types has been found leading back 
to a remote and very different progenitor. This sup- 
posed ancestor of the horse was a creature little larger 
than a moderate-sized dog. It had four separate toes 
to each fore-limb, and three to each hind-limb, and its 
teeth were much simpler and less specialized than 
those of existing horses. We say possibly ancestral, 
advisedly. Geological evidence is naturally insufficient 
to establish the actual relationship of the series of 
types which has been described, and Professor Sedg- 
wick has recently criticized the view that this series 
of forms constitutes a demonstrative historical proof 
of the doctrine of organic evolution. It is, therefore, 
preferable to claim this group of fossils as an illustra- 
tion of the possible geological ancestry of an existing 
species rather than as affording concrete proof of an 
actual pedigree. Even with this reservation, we claim 
that such a series constitutes a valuable collateral proof 
in favour of evolution. The general distribution of 
organisms throughout the geological strata agrees 
moreover, in a remarkable way with what is to be 
expected on the evolution theory. 
6. CHANGES UNDER DomESTICATION.—Among do- 
mesticated animals and plants we know of numerous 
cases in which the actual origin of new forms has been 
observed. These have often differed from their pre- 
decessors by amounts quite comparable with the dif- 
ferences by which natural species or even genera are 
separated. A notable example of this process is afforded 
by the numerous breeds of pigeons known to have arisen 
under domestication from a single wild species. We 
