CHANGE FACTORS. INTRODUCTION 311 



dwarfed in size. The relative sizes of the parts were unaltered, show- 

 ing that there had been no real struggle among the determinants; for, 

 on the theory of germinal selection, only the stronger determinants 

 would have survived and the weaker ones would have been starved 

 out. Partial individuals, moreover, lacking certain organs and over- 

 developed in others, would have been produced instead of individuals 

 merely smaller in all parts. 



These are the specific objections to the theory,but more important 

 than all of these is the general objection that follows: 



"Thus Weismann," says Morgan, 1 "has piled up one hypothesis 

 on another as though he could save the integrity of the theory of 

 natural selection by adding new speculative matter to it. The most 

 unfortunate feature is that the new speculation is skilfully removed 

 from the field of verification, and invisible germs whose sole functions 

 are those which Weismann's imagination bestows on them, are brought 

 forward as though they could supply the deficiencies of Darwin's 

 theory. This is, indeed, the old method of the philosophizers of 

 nature. An imaginary system has been invented which attempts to 

 explain all difficulties, and if it fails, then new inventions are to be 

 thought of. Thus we see where the theory of selection of fluctuating 

 germs has led one of the most widely known disciples of the Darwin- 

 ian theory. 



" The worst feature of the situation is not so much that Weismann 

 has advanced new hypotheses unsupported by experimental evidence, 

 but that the speculation is of such a kind that it is, from its very 

 nature, unverifiable, and therefore useless. Weismann is mistaken 

 when he assumes that many zoologists object to his methods because 

 they are largely speculative. The real reason is that the speculation is 

 so often of a kind that cannot be tested by observation and experiment." 



It seems almost impossible that the same Professor Morgan, who 

 wrote the foregoing paragraphs in 1903, should now be the leading 

 exponent of a theory of the mechanics of hereditary transmission 

 which depends upon hereditary units almost identical with Weis- 

 mann's "determinants," for the "genes" or "factors" of Morgan are 

 minute corporeal bodies in the germ cells which determine the charac- 

 ters of the adult individual. 



The difference is, however, that the "genes" of Morgan are experi- 

 mentally demonstrable and have behind them a vast amount of real 

 evidence for their existence. 



1 From T. H. Morgan, Evolution and Adaptation. 



