3 44 Problems of Organic Adaptation 



many germs, especially those affecting the respiratory tract. 

 It is known also that x-rays kill certain cells, particularly 

 the leucocytes, sooner than others. We know that it is pos- 

 sible to destroy parts of a cell and yet keep other parts 

 alive for a time at least; whole chromosomes may be lost 

 and yet the cell be capable of continued life and division. 

 What reason is there for supposing that the same may not 

 be true of the units of which chromosomes are composed, 

 and even of the genes themselves? If this should be true, 

 the elimination of the unfit may take place at any stage in 

 the ontogeny, and the least viable would be those which 

 disappear earliest and leave fewest traces. The greatest mis- 

 fits in the world never become visible to the naked eye, for 

 they never begin to develop. In this way doubtless many 

 mutations are eliminated before they ever come to light, 

 and so modifications which are disharmonious disappear al- 

 most as soon as they occur. 



Recent work of Morgan and his pupils shows that there 

 are inheritance factors or genes which are transmitted in 

 Mendelian fashion and which cause death either before 

 development begins or at some time during that process. 

 These "lethal factors" bring about the complete elimination 

 of certain genotypes, so that natural selection may be said 

 to begin in such cases with the genes themselves. But it may 

 be objected that such selection is not necessarily adaptive, 

 that it does not represent the survival of the fittest, since 

 these non-viable genotypes might have given rise to pheno- 

 types which were highly adapted to conditions of life if 

 only they could have lived; but it must not be forgotten 

 that in order to leave offspring organisms must live, and that 

 fitness to survive must be found not merely in adult stages 

 but in every stage leading up to the adult. Those individuals 

 that leave offspring must be fit at all stages of development 



