Translator s Preface vii 



pend upon ultimate verification, but is to be measured 

 by their effect upon scientific research. All this is now 

 a truism. 



What does it argue that no one, as Delage insists, 

 ever anticipated by imagination the striation of muscle 

 fibers, the existence of chromosomes and centrosomes, or 

 any other fact of minute structure revealed by the micro- 

 scope. May it not be asked in reply how long we should 

 have had to wait for the discovery of the inert gases of 

 the atmosphere, the accessory chromosome, and the ion, 

 had they not first been conceived in imagination and 

 formally embodied in working hypotheses? It is not 

 pleasant to contemplate what the effect on the develop- 

 ment of chemical science would have been had Dalton's 

 (micromeric) hypothesis of indivisible units been rejected 

 on the a priori grounds that the ultimate structure of 

 matter is beyond the power of the human intellect to 

 imagine in detail. 



The hypothesis of intracellular pangenesis can never 

 be absolutely demonstrated as true can never advance 

 beyond the rank of a theory because the hypothetical 

 pangens are conceived to be invisible, ultra-microscopic 

 units, whose existence can never be more than inferred; 

 but the formulation of the hypothesis marks the beginning 

 of the greatest and most important forward step in the 

 study of the origin of species since 1859. The notion 

 of pangens became the parent-idea of unit-characters, 

 offered a simple mechanism for the disjunction of char- 

 acters in hybrids, and for continuous and discontinuous 

 variation, and thus lead up directly to the conception of 

 mutation as one method of the origin of species.* And, 

 most important and significant of all, it resulted in per- 



*Cf. footnote, p. 74 infra. 



