September 23, 1904.] 



SCIENCE. 



399 



species, and asked for, another explanation 

 than that which was derived from the or- 

 dinary, slow and continuous variations. 



Their evidence, however, was not com- 

 plete enough to command the decision in 

 their behalf. The direct proof of the sud- 

 den changes could not be offered by them, 

 and they allowed themselves to be driven to 

 the acceptance of supernatural causes on 

 this account. Thereby, however, they lost 

 their influence upon the progress of science, 

 and soon fell into oblivion. 



Instead of following this historical line, 

 however, I have now to point out one of 

 the weightiest objections against the con- 

 ception of the origin of species by means 

 of slow and gradual changes. It is an ob- 

 jection which has been brought forward 

 against Darwin from the very beginning, 

 which has never relented, and which often 

 has threatened to impair the whole theory 

 of descent. It is the incompatibility of the 

 results concerning the age of life on this 

 earth, as propounded by physicists and as- 

 tronomers, 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, indi- 

 cate an age for the inhabitable 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 evo- 

 lution, from the very first beginning until 

 the appearance of mankind. 



This large discrepancy has always 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 much 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 

 amongst all, and much valuable discussion 

 and evidence has been brought together. 



The decision, however, could only be 

 gained by a direct study of the supposed 

 mutations, but no distinct cases of muta- 

 bility were at hand to provide the material. 

 Diseiissions 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 in- 

 quiry. Slow and gradual changes were 

 accepted to be invisible or nearly so; mu- 

 tations, however, would be clear and sharp, 

 although of rare occurrence. I determined 

 to start on a search for them, and tried a 

 large number of species, partly native 

 forms of my own country and partly from 

 different sources. Each of them had to be 

 tried as to its constancy, and large numbers 

 of seedlings had to be produced and com- 

 pared. The chance of finding what I 

 wanted was of course very small, and con- 

 sequently the number of the experiments 

 had to be increased as far as possible. 



Fortune has been propitious to me. It 

 has brought into my garden 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 mutability such as would be supposed 

 to occur in nature. The sudden changes, 

 which until yet were limited to the experi- 

 ence of the breeders, proved to be acces- 

 sible to direct experimental work. They 

 can not yet in truth be produced artificially, 



