394 THE EVIDENCE OF EVOLUTION. 



pounded by pliysicists and astronomers, with the demand made by the 

 theory of descent. 



The deductions made by Lord Kelvin and others from the central 

 heat of the earth, from the rate of the production of the calcareous 

 deposits, from the increase of the amount of salt in the water of the 

 seas, and from various other sources, indicate an age for the inhab- 

 itable surface of the earth of some millions of years only.. The most 

 probable estimates lie between twenty and forty millions of years. 

 The evolutionists of the gradual line, however, had supposed many 

 thousands of millions of years to be the smallest amount that would 

 account for the whole range of evolution, from the very first begin- 

 ning until the appearance of mankind. 



This large discrepancy has ahvays been a source of doubt and a 

 weapon in the hands of the opponents of the evolutionary idea, and it 

 is especially in this country that nuich good work has been done to 

 overcome this difficulty. The theory of descent had to be remolded. 

 On this point conviction has grown in America during the last 

 decades with increasing rapidity. Cope's works stand prominent 

 among all, and much valuable discussion and evidence has been 

 l^rought together. 



The decision, however, could onlv be gained by a direct study of the 

 supposed mutations, but no distinct cases of mutability were at hand 

 to provide the material. Discussions took the place of inquiry, and 

 a vast amount of literature has broadly pictured all the possibilities 

 and all the more or less plausible explanations without being able 

 to give proof or disproof. 



In this most discouraging state of things I concluded that the only 

 way to get out of the prevailing confusion was to return to the 

 method of direct experimental inquiry- Sknv and gradual changes 

 were accepted to be invisible or nearly so; mutations, however, 

 would be clear and sharp, although of rare occurrence. 1 deter- 

 mined to start on a search for them, and tried a large number of 

 species, partly native forms of my own country and partly from 

 difterent sources. Each of them had to be tried as to its constancy, 

 and large numbers of seedlings had to be produced and compared. 

 The chance of finding what I wanted was of course very small, and 

 consequently tlie number of the experiments had to be increased as 

 far as possible. 



Fortune has been jn-opitious to me. It has brought into my gar- 

 den a series of mutations of the same kind as those which are known 

 to occur in horticulture, and moreover it has afforded me an instance 

 of nnitability such as would be supposed to occur in nature. The 

 sudden clianges, which until yet were limited to the exi^erience of the 

 breeders, proved to be accessible to direct experimental work. They 

 can not yet in truth be produced artificially, but, on the other hand, 



