260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910 



million years to reduce the four effective digits of Eohippus (Lower 

 Eocene) to three in Mesohippus (Lower Oligocene), and 22 million 

 more to raise the two lateral digits clear of the ground (in Mery chip- 

 pus, Middle Miocene). The penultimate stage of reducing the in- 

 effective side digit to vestigial splints (in Flesippus, Upper Pliocene) 

 occupied some 16 million years, and the gradual reduction of these 

 vestiges to the condition seen in the modern horse {Equus, Upper 

 Pliocene) probably took about another 2 million more. 



Time evolution in the series of invertebrate animals may be illus- 

 trated from A. W. Rowe's well-known study (1899) of the unbroken 

 gradation of variations in the fossil sea urchin Micraster. The Cre- 

 taceous period occupied in all about 40 million years. A rough pro- 

 portional estimation would suggest that the evolution of the founda- 

 tion species of the series Micraster cor-hovis into M. cortestudinariwm 

 (i. e., from the Terebratula gracilis zone to Holaster planus zone) 

 took about 1 million years, while the further changes which eventually 

 produced Micraster cor-anguinum occupied an additional 1% million 

 years. 



From another standpoint a view may be gained of the race of 

 evolution in recent times. The American scientist, O. P. Hay, stated 

 in 1928 that "a learned writer on mammals tells me that he doubts 

 that a single new species has developed since the first interglacial 

 stage." But the suggestion is incorrect. In the outer islands of 

 Scotland there are recognized several species which are distinctive 

 of the islands, such as the Islay shrew {Sorex granti), the Orkney 

 vole (Microtus orcadensis) , the Mull bank vole {Evotomys alstoni), 

 the Hebridean field mouse {Apodemus kehridensis) , or its relatives 

 in Fair Isle (A. fridariensis) and in St. Kilda {A. hirtensis) , and the 

 St. Kilda house mouse {Mus Tnuralis) — to take examples from sev- 

 eral different genera. We need not discuss the special characters of 

 these species, it is enough that they are recognizable and are regarded 

 as distinctive by the specialists best qualified to judge. 



Now a time limit is set to the evolution of these species. Scotland 

 itself and its islands were overwhelmed with glaciers during the Ice 

 Age, life was extinguished, and it was only after the ice had passed 

 away and the land had subsequently become clothed with vegetation 

 that the country became fit for even shrews and rodents to live in. 

 The mainland of Scotland was stocked by migration from the con- 

 tinent of Europe, the islands presumably from the mainland, and 

 since that time the new species of the islands have branched off from 

 their mainland ancestors. All this must have happened in a period 

 subsequent to the last gTeat glaciation. Penck reckons the Blihl 

 stadium of postglacial time in Northern Europe at about 20,000 years 

 B. C, and that probably indicates the maximum of time which may 

 be allowed for the evolution of the Scottish island species. 



