DEGENERATION THROUGH SURVIVAL OF INFERIOR INDIVIDUALS. 73 



same mutation should become the parents of two isolated colonies, it 

 ought to predict that every variation occurring in cither colony will 

 be found in both, and in the same proportion. That this will be the 

 case has not yet been shown. 



Without variation in its different forms cooperating with heredity, 

 isolation and selection could have no influence in guiding and shaping 

 evolution. It is, therefore, well that the mutation theory insists on the 

 importance of these fundamental factors and of the laws by which 

 they are controlled. But it must be remembered that through the 

 powers of variation and heredity the principles of free crossing, isola- 

 tion, and selection gain profound significance. These latter are the 

 conditions through which the laws of the fundamental factors arc 

 revealed ; and if we misinterpret the laws we shall fail in our explana- 

 tion of the process. These laws have not been fully brought to light ; 

 but is there no reason to suspect that there is something lacking in 

 the theory that the selection of individual variations can have no 

 effect on the final result? * 



18. Degeneration of Species when the Standard of Survival is Lowered. 



The frequently observed fact that characters built up by the arti- 

 ficial selection of individual variations gradually disappear when the 

 selection entirely ceases seems to be the chief reason for disregarding 

 the influence of such variations in the formation of varieties and 

 species. But is not the same tendency seen in the characters of nat- 

 ural species which the mutation theory assumes to have resulted from 

 mutations? Are there not characters that have been maintained 

 with unbroken constancy through countless generations of ancestors, 

 not only through all the past history of the present species, but 

 through the much longer history of many ancestral species, and that 

 yet do not reach, in every individual of the species, the standard neces- 

 sary for survival? If such individuals are able to escape the fate that 

 has overtaken similarly defective ones of previous generations they 

 will help to lower the standard of attainment previously gained by 

 the species to which they belong. This process may be repeated in 

 successive generations till the character is entirely lost. Different 

 stages of such a process are revealed in the present condition of cer- 

 tain species of birds, in regard to the instinct that leads the mother 

 bird to sit on the eggs she lays, and to provide for the young when 



* Prof. T. H. Morgan's position is seen in the following statement: "Nature's 

 supreme test is survival. She makes new forms to bring them to this test through 

 mutation, and does not remodel old forms through a process of individual selec- 

 tion." See "Evolution and Adaptation," p. 464. 



