l. c. cooley. 59-(29) 



thesis of these hydrocarbons have become common, and 

 are no longer received with surprise. Of organic com- 

 pounds which contain oxygen and nitrogen a smaller 

 number are laboratory products. It is in the manufac- 

 ture of these that chemical synthesis has won its latest 

 and most brilliant triumphs, and yet its trophies are few 

 and insignificant when compared with the multitudes of 

 more complex compounds of the same class which it has 

 not been able to reach. 



Of the proximate constituents of plants and animals 

 very few indeed are artificial products. Acetic acid is 

 found ready-made, in the juices of some ; it has been 

 also prepared from inorganic matter in the laboratory. 

 Other examples might be given. But the efforts of chem- 

 ical synthesis have been unsuccessful beyond a small 

 number of the simplest substances which are known to 

 exist in organized bodies, while, as it is almost needless 

 to say, not even the simplest among organized bodies 

 themselves has ever appeared as an artificial product, 

 Balancing the account we find that a large number of 

 the products of chemical decomposition of organic 

 substances, and a very few of the simplest proximate 

 constituents of organized bodies stand to the credit of 

 the synthetical chemist, while over against these stand 

 the whole realm of organized bodies, together with all 

 their complex and nearly the whole number of their 

 simpler constituents. 



Now all will admit that the hypothesis of a vital force 

 has retreated just as far as chemical synthesis has actu- 

 ally advanced. The question is how far shall chemical 

 synthesis be allowed to project its inferences into terri- 

 tory which it has not yet subdued. 



I hold it to be the first principle in the philosophy of 

 science,, that a generalization must be confined to objects 

 of the same class and under similar conditions as those 

 covered by the specific observations on which the gener- 

 alization is based. In the scientific harvest, as elsewhere. 

 one must not attempt to gather figs from thistles. Ob- 

 servations and experiments contain the germs of general 

 principles, but each can bear fruit only after its kind. I 

 am persuaded that a degree of carelessness in this par- 

 ticular is a most prolific source of error. Slight analo- 

 gies sometimes usurp the place of identities, and impair 



