PERSPECTIVES IN EVOLUTION — RITCHIE 259 



800 million years, and stretching away beyond that we must imagine 

 a period when life was evolving in its most primitive forms, of which 

 no trace remains or could remain. We may say that life has existed 

 upon the earth for perhaps 1,200 millions of years; and then to com- 

 plete the picture that the birth of the earth, and as the new cos- 

 mology seems to indicate, perhaps also at the same time the 

 stupendous birth of sun and stars, took place about 2,000 million 

 years ago. 



STABILITY OF ORGANISM AND SLOWNESS OF EVOLUTION 



This amazing extension of the time concept of life emphasizes anew 

 some of the striking features of evolution. We are accustomed to lay 

 stress on the variation of living things, upon which evolution depends, 

 but surely more remarkable is the stability of living organisms, which 

 retain their own characters in spite of changes in the environment, 

 and whose germ cells pass these characters unaltered through count- 

 less generations. The edible cockle {Cardiumi edule) has retained its 

 specific characters for 2 million years or more; its genus, in a wide 

 sense, lived 160 million years ago in the Trias. The crinoid genus 

 Antedon which flourishes in our own seas antedated that old bird 

 Archaeopteryx in the Jurassic period, 140 million years ago. It is 

 surprising enough to realize that genera of foraminifera, like 

 Nodosaria (Silurian) and Saccanrniina (Ordovician), still abundant 

 in our oceans, have retained their generic characters for about 300 

 million years. But they are relatively simple organisms; it is still 

 more astonishing to think that contemporaneous with them or before 

 them lived modern genera (again in the wide sense) of more highly 

 organized brachipods, like Lingida (Ordovician), and Crania (Ordo- 

 vician), and that these have experienced the geologic upheavals and 

 secular changes since Paleozoic times without turning a hair, or in 

 the revised version, without the shift of a gene upon a chromosome. 



It is in agreement with that stability of organisms that we must 

 conceive of evolution as a process of extreme slowness, as if living 

 things are loath to change, and ultimately change only under the direct 

 compulsion of circumstances. Of that slow progress in its minor 

 phases the new chronology gives us a measure. One or two well- 

 known examples will illustrate the point. Matthew (1914) and Osborn 

 (1917), each in his day, suggested a time scale for the evolution of 

 the modern horse (Equus) from its precursors of Eocene times. But 

 the newer data bearing upon geologic ages should contribute further 

 precision to a fresh estimate. Accepting, then, the data from helium 

 and lead methods of time estimation as given by Holmes (1937), we 

 find that the whole gamut of changes which modified the four-toed 

 f orelimb of Eohippus into the single toe of Equus, from Lower Eocene 

 to Upper Pliocene, occupied about 57 million years. It took some 17 



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