E VIDENCES OF E VOL UTION, 129 



rudimentary metacarpal bone of a fourth digit in 

 the fore-foot ; in mesohippus this rudimentary meta- 

 carpal bone is more fully developed ; in orohippus 

 there are four well-developed digits in the fore-foot, 

 three in the hind-foot ; while in eohippus five digits 

 are present. Thus, this series of fossil forms fur- 

 nishes a complete gradation, from the older Tertiary 

 forms with four toes, up to the horse with one toe. 

 These forms differ not only as regards the number 

 of toes, but also in other respects, chiefly in the 

 gradual diminution and loss of independence of the 

 ulna and fibula, and in the gradual elongation of the 

 teeth and increasing complexity of the grinding 

 surfaces." ' 



Another interesting example frequently cited, of 

 transitionary forms, is the fossil, planorbis, found in 

 the bed of an old lake near the small village of 

 Steinheim, in Wurtemberg. In the successive strata 

 of this lake bottom occur an immense number of 

 shells of divers forms, and all from a few varieties 

 of one and the same species. In passing from the 

 lowest to the highest layers a great modification of 

 forms is observed, so much so, indeed, that were it 

 not for the countless intermediate forms one should 

 unhesitatingly say that the extreme forms belong, 

 not only to different species, but even to different 

 genera. As it is, however, the gradations are so in- 

 sensible that the conclusion is almost irresistible 



1 " Lectures on the Darwinian Theory," by Dr. A. M. Mar- 

 shall, p. 67. For an interesting discussion with diagrams, of 

 this remarkable series of ancestral equine forms, see the third of 

 Huxley's " Lectures on Evolution," entitled The Demonstra- 

 tive Evidence of Evolution. 



E.-9 



