360 PRINCIPLES OF ANIMAL BIOLOGY 



be an aid in the struggle for existence. One gained the idea, from follow- 

 ing the arguments of these enthusiasts, that animals possessed no traits 

 that were neutral or harmful, but that every structure and every habit 

 was adapted to some use, and that the present existence of these 

 structures and habits was due to their usefulness. It is now known, 

 however, that such useless or detrimental characters are not uncommon. 

 Through no fault of Darwin's then, but because of the zeal of his apostles, 

 the theory of natural selection came into a degree of neglect. Although 

 careful breeding experiments indicate the effectiveness of artificial selec- 

 tion, there is no marked inclination among leading biologists to attribute 

 to selection in nature, through the struggle for existence, anything but a 

 minor and negative role in evolution. Any modification that is very 

 harmful would, of course, probably cause the death of its possessor. 

 But characters that are neither useful nor harmful as well as those sUghtly 

 harmful and those that are beneficial, might well be preserved. If the 

 destruction of individuals in which very harmful new traits have appeared 

 is all that natural selection accomplishes, it plays a much less important 

 part in directing the course of evolution than Darwin (to say nothing of 

 his more zealous and less discriminating followers) supposed. 



Direction of Mutation. —Such negative action by natural selection 

 could not, however, have been responsible for the course of evolution 

 in the development of modern animals from their early ancestors. The 

 direction of this great positive development, with httle help or hin- 

 drance from natural selection, must have been due to something else. 

 That other agency is believed to lie in the native internal capacity 

 of protoplasm for undergoing change, or producing mutations, which was 

 discussed among the causes of evolution but which must be alluded to 

 here also as a factor determining the direction of evolution. Since muta- 

 tions, as pointed out in the previous discussion, are probably the result of 

 chemical changes of the chromatin, it will be seen that although a large 

 number of such changes may be possible, there is a limit both to the 

 number and to the kind of changes that may occur, and that the direc- 

 tion of evolution must be one or more of the directions of these possible 

 changes. The kind of chemical change that occurs in a protein, for 

 example, is strictly limited by the structure of its molecule. The kind of 

 mutation that occurs, and hence the direction of evolution, is in turn 

 dependent upon the kind of chemical change that is possible in the chro- 

 matin of the germ cells. The direction of evolution is therefore deter- 

 mined by the nature of the chromatin. 



Orthogenesis. — What direction the changes in the chromatin, and 

 hence the changes in the adult, will take cannot be foretold. It is 

 possible that the molecular changes of the chromatin take place in a 

 purely random manner. On the contrary, the shifting of the atoms in 

 a particular manner at one time may make a further change of the same 

 kind at a later time more probable. If the latter supposition is correct, 



