The Mechanism of Adaptation 343 



nomena of evolution, and especially of the degeneration and 

 disappearance of useless parts and the concordant modifi- 

 cation of numerous parts of the organism. In order to 

 explain these he proposed to extend the principle of natural 

 selection from individuals or persons ("personal selection" 

 or "Darwinism" in the strict sense) to organs and tissues 

 ("histonal selection" of Roux), and even to germinal units 

 such as determinants and biophores ("germinal selection"). 

 This hypothesis as originally proposed was open to many 

 and serious objections. It is impossible to hold with Weis- 

 mann that there is a struggle between germinal elements for 

 food, and that the weaker ones are starved and eliminated 

 in this struggle; but we are on safe ground when we affirm 

 that natural selection is operative at every stage in develop- 

 ment from the earliest steps in the formation of the germ- 

 cells up to the adult condition. Not even the most radical 

 critic of Darwinism doubts that animals which cannot live 

 die. No one doubts that this is true also of individual cells 

 as well as of persons: What reason is there to suspect that 

 it is not also true of parts of cells, such as plastids, nuclei, 

 chromosomes, chromomeres, and even genes? We know 

 that many young forms perish before reaching maturity, 

 that numerous organisms never develop beyond embryonic 

 stages, that multitudes of germ-cells perish, and that, in 

 general, elimination is much more severe in the earlier than 

 in the later stages of ontogeny. We know that in the life of 

 higher organisms many kinds of cells are continually dying 

 and being replaced by others; so far as epithelial, glandular, 

 and blood cells are concerned, we may say with St. Paul, 

 "We die daily." The death of cells is frequently selective; 

 for example, it is said by medical authorities that the leuco- 

 cytes or white blood cells are destroyed in large numbers by 

 the influenza germ, thus opening the way to infection by 



