The Unity of the Organism 



facts discovered and principles laid down by Wolff and von 

 Baer, and the truth expressed has been the foundation of 

 all solid achievement in embryology throughout the history 

 of the science. It is strange that conditions should arise 

 at this time of advanced progress in biology which involve 

 what seems decidedly like an abandonment of this founda- 

 tion. 



It could hardly have been believed in the hey-day of 

 descriptive embryology, in the decade, for instance, follow- 

 ing the publication of Francis Balfour's "Comparative Em- 

 bryology" that the time would again come when the domi- 

 nant theories of organic genesis would have regard to the 

 completed organism at one end of the series, and the germ- 

 inal elements at the other, with well-nigh complete neglect 

 of all the intervening stages. Yet this is essentially what 

 has happened. One cannot avoid seeing this if he examines 

 with open mind almost any of the literature produced by 

 the modern school of geneticists, especially if the work has 

 been under the spell of Mendelism accepted as a creed to 

 which conformity must be reached by hook or crook, and not 

 as an instrument to be used when applicable and useful. 

 Unless observation is to be denied a primary place in bio- 

 logical method, and unless that place can be unreservedly 

 given over to inference and deduction, I see no escape from 

 the necessity of testing by the familiar methods of embryol- 

 ogy the hypothesis that chromosomes or any other particu- 

 lar bodies are the mechanism of heredity; that is, of showing 

 in what particular way the bodies participate in the origin 

 of a given part or organ from its forerunners in the develop- 

 mental series : If, for example, it be contended that a par- 

 ticular portion of a chromosome causally explains eye color, 

 then some activity or transformation, morphological or 

 chemical, of that chromosome, not away back in the germ- 

 cell, but in the cells of the part from which the color-bearing 

 cells immediately arise, should be proved. This alone would 



