do not cease to be because we ignore them. The paradoxes into which 

 the biologists fall, in their efforts to locate the substance of inherit- 

 ance, remind me of the perplexity of the school-boy, who, having tried 

 to add together six cows and nine horses and four apples, wonders 

 whether the result is horses or cows or apples. If he were to attribute 

 the virtue of arithmetic to a substance of numeration, and to wonder 

 whether it resides in apples or in cows, he would be still more like 

 those who speculate about the location of the substance of inheritance. 



If you choose to declare that my contention that inheritance is not 

 a fact is a metaphysical subtilty, I cannot help it. Call me a metaphy- 

 sician if you will. Hard words cannot hurt me, nor need they scare 

 me. But may it not be the speculative zoologist, who hunts in germ- 

 cells and in chromatin for the material support of the imperfection 

 of his ideas, who is the real metaphysician, and not I, who plead for 

 nothing but the correction of our judgment, and its reduction to 

 exactness, by comparison with nature? 



Some embryologists tell us that all the cells that enter into the 

 structure of a multicellular organism are inherently identical with 

 each other, and with the fertilized egg, in their constitution, and in 

 the possibilities of their development. Each, we are told, might have 

 replaced any other if it had been exposed to the same conditions, and 

 might have become a germ-cell under proper conditions. According 

 to this view, cell-division is always division into parts that are iden- 

 tical with each other, and with the cell that gave rise to them, and it 

 is only because they are exposed to different conditions that cells 

 become specialized and differentiated during development, and not 

 because there is any inherent difference between them. According 

 to this way of looking at individual development, any attempt to 

 account for it by the imaginary architecture of the germ or of its 

 chromatin is idle, because the architecture of the organism does not 

 exist in the germ, since it is the resultant of the reciprocal interaction 

 between the germ and the developing embryo with the conditions 

 of their existence. You will permit me to say that, with certain quali- 

 fications and reservations, this view of the nature of individual 

 development commends itself to me as a step in the right direction. 

 The correlation between the normal development of one part of the 

 body and the development of other parts is a familiar fact. The 

 changes that take place in the habits and the plumage of certain 

 birds, as the reproductive organs become functional on the approach 

 of the breeding-season, are known to all, as is also the arrest of these 

 changes when the reproductive organs are removed or aborted by 

 disease. The awakening of the body into full and rounded perfection 

 which comes with the functional maturity of the reproductive organs 

 is so notable in our most familiar mammals that it has come to be 

 regarded as the typical illustration of the correlation between de- 



